exportfs: update Exporting documentation

Minor documentation updates:
	- refer to d_obtain_alias rather than d_alloc_anon
	- explain when to use d_splice_alias and when
	  d_materialise_unique.
	- cut some details of d_splice_alias/d_materialise_unique
	  implementation.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>