fs: Add more kernel-doc to the produced documentation

People have gone to all the effort of writing kernel-doc for these
functions; the least we can do is put them in the "Other functions"
part of the VFS documentation.

Signed-off-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>