lib/stackdepot: move documentation comments to stackdepot.h

Move all interface- and usage-related documentation comments to
include/linux/stackdepot.h.

It makes sense to have them in the header where they are available to
the interface users.

[akpm@linux-foundation.org: grammar fix, per Alexander]
Link: https://lkml.kernel.org/r/fbfee41495b306dd8881f9b1c1b80999c885e82f.1676063693.git.andreyknvl@google.com


Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Alexander Potapenko <glider@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>