Add link for boundary conditions in -n (#8589)

doc/rst/source/explain_-n_full.rst_

@@ -22,7 +22,8 @@ The following modifiers are supported:
 - **+a** - Switch off antialiasing (where supported) [default uses antialiasing].
 - **+b** - Override boundary conditions used, by appending *g* for geographic, *p* for periodic, or *n* for
   natural boundary conditions. For the latter two you may append **x** or **y** to specify just one direction, otherwise
-  both are assumed.
+  both are assumed. For more information see :ref:`grid-boundary-conditions`.
+
 - **+c** - Clip the interpolated grid to input z-min/z-max [default may exceed limits].
 - **+t** - Control how close to nodes with NaNs the interpolation will go based on *threshold*. A *threshold* of 1.0
   requires all (4 or 16) nodes involved in interpolation to be non-NaN. For example, 0.5 will interpolate about half

doc/rst/source/reference/file-formats.rst

@@ -263,14 +263,16 @@ option (see Section :ref:`option_nodereg`). **Note**: The smallest
 pixel-registered grid can be 1x1 (storing a single value), while a
 gridline-registered grid cannot be smaller than 2x2.
 
+.. _grid-boundary-conditions:
+
 Boundary Conditions for operations on grids
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 GMT has the option to specify boundary conditions in some programs
 that operate on grids (e.g.,
 :doc:`/grdsample`, :doc:`/grdgradient`,
 :doc:`/grdtrack`, :doc:`/nearneighbor`, and
-:doc:`/grdview`, to name a few. The desired
+:doc:`/grdview`, to name a few). The desired
 condition can be set with the common GMT option **-n**; see Section
 :ref:`option_-n`. The boundary conditions come into play when
 interpolating or computing derivatives near the limits of the region