Documentation/zram: add documentation for algorithm parameters

Document brief description of compression algorithms' parameters:
compression level and pre-trained dictionary.

[senozhatsky@chromium.org: trivial fixup]
  Link: https://lkml.kernel.org/r/20240903063722.1603592-1-senozhatsky@chromium.org
Link: https://lkml.kernel.org/r/20240902105656.1383858-24-senozhatsky@chromium.org


Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Nick Terrell <terrelln@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>