kref: Improve documentation

kref: Improve documentation

There is already kernel-doc written for many of the functions in kref.h
but it's not linked into the html docs anywhere.  Add it to kref.rst.

Improve the kref documentation by using the standard Return: section,
rewording some unclear verbiage and adding docs for some undocumented
functions.

Update Thomas' email address to his current one.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20241209160953.757673-1-willy@infradead.org