binutils/doc: Update documentation for nm --size-sort

binutils/doc: Update documentation for nm --size-sort

The documentation for 'nm --size-sort' is out of step with the code, the
special handling for ELF is not mentioned in the documentation, and
could cause confusion to a user.

binutils/ChangeLog:

	* doc/binutils.texi (nm): Update description of --size-sort.