lib/string_helpers: fix some kerneldoc warnings

lib/string_helpers: fix some kerneldoc warnings

Due to some sad limitations in how kerneldoc comments are parsed, the
documentation in lib/string_helpers.c generates these warnings:

  lib/string_helpers.c:236: WARNING: Unexpected indentation.
  lib/string_helpers.c:241: WARNING: Block quote ends without a blank line; unexpected unindent.
  lib/string_helpers.c:446: WARNING: Unexpected indentation.
  lib/string_helpers.c:451: WARNING: Block quote ends without a blank line; unexpected unindent.
  lib/string_helpers.c:474: WARNING: Unexpected indentation.

Rework the comments to obtain something like the desired result.

Link: http://lkml.kernel.org/r/20190607110952.409011ba@lwn.net
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>