doc: clarify current realpath --relative-base behavior

doc: clarify current realpath --relative-base behavior

For compatibility with MacOS relpath(1), as seen here:

http://opensource.apple.com/source/bootstrap_cmds/\
bootstrap_cmds-79/relpath.tproj/relpath.c

we implemented 'realpath --relative-base=dir1 --relative-to=dir2 file'
in the same way as 'relpath -d dir1 dir2 file'.  This can result
in --relative-base rendering --relative-to as a no-op if dir1 is a
child of dir2.  Document this.

* doc/coreutils.texi (realpath invocation): Mention restriction.