kernel-doc: Handle returning pointers to pointers

Clearly nobody ever tried to build the documentation for the radix tree
before:

include/linux/radix-tree.h:400: warning: cannot understand function
prototype: 'void ** radix_tree_iter_init(struct radix_tree_iter *iter,
unsigned long start) '

Indeed, the regexes only handled a single '*', not one-or-more.  I have
tried to fix that, but now I have perl regexes all over my hands, and
I fear I shall never be clean again.

Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>