doc: clarify when cp and mv output xattr warnings
The 2010-03-26 commit,
4c38625e, "doc: fix info on cp --preserve..."
was not entirely correct as cp --preserve=all does produce some
xattr warnings.
* src/copy.h: Update and clarify the comments for reduce_diagnostics
and require_preserve_{xattr,context}.
* doc/coreutils.texi (cp invocation): Update the -a and
--preserve=xattr,context options to say when and which
xattr warnings are output.
(mv invocation): Mention that some warnings are output
when preserving xattrs.