Document that zstd:chunked doesn’t make sense with encryption

[mtrmac]

Encrypted images don’t benefit from zstd:chunked , and if asked to do so, the code uses non-chunked zstd (ref. containers/image#2485 .)

Around

common/pkg/config/containers.conf

Line 442 in 1a8d191

#compression_format = "gzip"

, and for add_compression option: the documentation in comments and/or man pages [wherever the authoritative source is] should probably note that zstd:chunked is not used for encrypted images [and triggers warnings].