maple_tree: fix a few documentation issues

The documentation of mt_next() claims that it starts the search at the
provided index.  That's incorrect as it starts the search after the
provided index.

The documentation of mt_find() is slightly confusing.  "Handles locking"
is not really helpful as it does not explain how the "locking" works. 
Also the documentation of index talks about a range, while in reality the
index is updated on a succesful search to the index of the found entry
plus one.

Fix similar issues for mt_find_after() and mt_prev().

Reword the confusing "Note: Will not return the zero entry." comment on
mt_for_each() and document @__index correctly.

Link: https://lkml.kernel.org/r/87ttw2n556.ffs@tglx


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Shanker Donthineni <sdonthineni@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>