doc: enhance diagnostic when rm skips "." or ".." arguments
The error diagnostic
"rm: cannot remove directory: '.'"
does not give the user a hint for the reason.
Issue a clearer error message.
* src/remove.c (rm_fts): Enhance the error diagnostic in the above
case to emphasize that skipping is done deliberately.
In the corresponding comment, mention that POSIX mandates this
behavior. Likewise in the subsequent comment for skipping "/".
* doc/coreutils.texi (rm invocation): In the paragraph describing
the above behavior, mention that POSIX mandates it.