Release notes for Unidata#2176, format compatibility

* Add current release note for Unidata#2176.
* Add missing release note for Unidata#951 in last release 4.8.1.
* Improve related old note for release 4.4.1.  Better consistency with recent updates.

RELEASE_NOTES.md

@@ -7,6 +7,7 @@ This file contains a high-level description of this package's evolution. Release
 
 ## 4.8.2 - TBD
 
+* [File Change] Apply HDF5 v1.8 format compatibility when writing to previous files, as well as when creating new files.  The superblock version remains at 2 for newly created files.  Full backward read/write compatibility for netCDF-4 is maintained in all cases.  See [Github #2176](https://github.com/Unidata/netcdf-c/issues/2176).
 * [Bug Fix] Clean up the handling of deeply nested VLEN types. Marks nc_free_vlen() and nc_free_string as deprecated in favor of ncaux_reclaim_data(). See [Github #2179](https://github.com/Unidata/netcdf-c/pull/2179).
 * [Bug Fix] Make sure that netcdf.h accurately defines the flags in the open/create mode flags. See [Github #2183](https://github.com/Unidata/netcdf-c/pull/2183).
 * [Enhancement] Improve support for msys2+mingw platform. See [Github #2171](https://github.com/Unidata/netcdf-c/pull/2171).
@@ -47,6 +48,7 @@ This file contains a high-level description of this package's evolution. Release
 * [Enhancement] Add support for reading/writing pure Zarr storage format that supports the XArray _ARRAY_DIMENSIONS attribute. See [Github #1952](https://github.com/Unidata/netcdf-c/pull/1952) for more information.
 * [Update] Updated version of bzip2 used in filter testing/functionality, in support of [Github #1969](https://github.com/Unidata/netcdf-c/issues/1969).
 * [Bug Fix] Corrected HDF5 version detection logic as described in [Github #1962](https://github.com/Unidata/netcdf-c/issues/1962).  
+* [File Change] Change from HDF5 v1.6 format compatibility, back to v1.8 compatibility, for newly created files.  The superblock changes from version 0 back to version 2.  An exception is when using libhdf5 deprecated versions 1.10.0 and 1.10.1, which can only create v1.6 compatible format.  Full backward read/write compatibility for netCDF-4 is maintained in all cases.  See [Github #951](https://github.com/Unidata/netcdf-c/issues/951).
 
 ## 4.8.0 - March 30, 2021
 
@@ -165,7 +167,7 @@ See [GitHub #1251](https://github.com/Unidata/netcdf-c/issues/1251).
 * Fix of NULL parameter causing segfaults in put_vars functions. See [Github #1265](https://github.com/Unidata/netcdf-c/issues/1265) for more information.
 * Fix of --enable-benchmark benchmark tests [Github #1211](https://github.com/Unidata/netcdf-c/issues/1211)
 * Update the license from the home-brewed NetCDF license to the standard 3-Clause BSD License.  This change does not result in any new restrictions; it is merely the adoption of a standard, well-known and well-understood license in place of the historic NetCDF license written at Unidata.  This is part of a broader push by Unidata to adopt modern, standardized licensing.
-* [BugFix] Corrected DAP-releated issues on big-endian machines. See [Github #1321](https://github.com/Unidata/netcdf-c/issues/1321), [Github #1302](https://github.com/Unidata/netcdf-c/issues/1302) for more information.
+* [BugFix] Corrected DAP-related issues on big-endian machines. See [Github #1321](https://github.com/Unidata/netcdf-c/issues/1321), [Github #1302](https://github.com/Unidata/netcdf-c/issues/1302) for more information.
 * [BugFix][Enhancement]  Various and sundry bugfixes and performance enhancements, thanks to \@edhartnett, \@gsjaardema, \@t-b, \@wkliao, and all of our other contributors.
 * [Enhancement] Extended `nccopy -F` syntax to support multiple variables with a single invocation. See [Github #1311](https://github.com/Unidata/netcdf-c/issues/1311) for more information.
 * [BugFix] Corrected an issue where DAP2 was incorrectly converting signed bytes, resulting in an erroneous error message under some circumstances. See [GitHub #1317](https://github.com/Unidata/netcdf-c/issues/1317) for more information.  See [Github #1319](https://github.com/Unidata/netcdf-c/issues/1319) for related information.
@@ -273,7 +275,7 @@ disabled until the testserver situation is resolved.
 
 ## 4.4.1 - June 28, 2016
 
-* [File Change] Starting with release 4.4.1, netCDF-4 files created will have superblock version 0 instead of superblock version 2, as was observed in previous netCDF versions.  This is due to a workaround required to avoid backwards binary incompatibility when using libhdf5 1.10.x or greater.  Superblock versions 0 and 2 appear to be forward and backward compatible.  Other than a different superblock number the data should remain consistent.
+* [File Change] Starting with release 4.4.1, netCDF-4 files are created with HDF5 v1.6 format compatibility, rather than v1.8.  The superblock changes from version 2, as was observed in previous netCDF versions, to version 0.  This is due to a workaround required to avoid backwards binary incompatibility when using libhdf5 1.10.x or greater.  Superblock versions 0 and 2 appear to be forward and backward compatible for netCDF purposes.  Other than a different superblock number the data should remain consistent.  See [GitHub #250](https://github.com/Unidata/netcdf-c/issues/250).
 * [Enhancement] Added better error reporting when ncdump/nccopy are given a bad constraint in a DAP url. See [GitHub #279](https://github.com/Unidata/netcdf-c/pull/279) for more information.
 
 ### 4.4.1-RC3 - June 17, 2016