Documentation/sysctl: document page_lock_unfairness

commit 5ef64cc8

 ("mm: allow a controlled amount of unfairness in the
page lock") introduced a new systctl but no accompanying documentation.

Add a simple entry to the documentation.

Link: https://lkml.kernel.org/r/20220325164437.120246-1-jsavitz@redhat.com
Signed-off-by: Joel Savitz <jsavitz@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: "zhangyi (F)" <yi.zhang@huawei.com>
Cc: Charan Teja Reddy <charante@codeaurora.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>