doc: rmdir: clarify --ignore-fail-on-non-empty operation

doc: rmdir: clarify --ignore-fail-on-non-empty operation

This option has changed from ignoring only ENOTEMPTY|EEXIST
(i.e. ignore errors _solely_ due to dir not empty),
to ignoring some other errors from more protected dirs
that are not empty.  That adjustment was made to better
support use with --parents, to essentially remove as much of
a hierarchy as possible, without erroring as we hit more
protected non empty parent dirs.
That functionality adjustment was originally discussed at:
https://lists.gnu.org/r/bug-coreutils/2008-01/msg00283.html

* src/rmdir.c (usage): Adjust to be more accurate to current behavior.
Also adjust --parents option to be easier to read.
* doc/coreutils.texi (rmdir invocation): Likewise.
Reported at https://github.com/coreutils/coreutils/issues/40