docs: path-lookup: update WALK_GET, WALK_PUT desc

docs: path-lookup: update WALK_GET, WALK_PUT desc

WALK_GET is changed to WALK_TRAILING with a different meaning.
Here it should be WALK_NOFOLLOW. WALK_PUT dosn't exist, we have
WALK_MORE.

WALK_PUT == !WALK_MORE

And there is not should_follow_link().

Related commits:
commit 8c4efe22e7c4 ("namei: invert the meaning of WALK_FOLLOW")
commit 1c4ff1a87e46 ("namei: invert WALK_PUT logics")

Signed-off-by: Fox Chen <foxhlchen@gmail.com>
Reviewed-by: NeilBrown <neilb@suse.de>
[jc: applied language tweaks suggested by Neil]
Link: https://lore.kernel.org/r/20210527091618.287093-11-foxhlchen@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>