Update flags documentation

Update flags documentation with information on when DONT_SHALLOW_CLONE
could be useful.

Author: gorshunovr

docs/guides/feature-flags.rst

@@ -27,3 +27,7 @@ Available Flags
 ``MKDOCS_THEME_RTD``: :featureflags:`MKDOCS_THEME_RTD`
 
 ``DONT_SHALLOW_CLONE``: :featureflags:`DONT_SHALLOW_CLONE`
+
+The ``DONT_SHALLOW_CLONE`` flag is useful if your code accesses old commits during docs build,
+e.g. python-reno release notes manager is known to do that
+(error message line would probably include one of old Git commit id's).