Add short documentation on every MRF Policy

lib/pleroma/web/activity_pub/mrf/anti_followbot_policy.ex

lib/pleroma/web/activity_pub/mrf/drop_policy.ex

lib/pleroma/web/activity_pub/mrf/ensure_re_prepended.ex

lib/pleroma/web/activity_pub/mrf/hellthread_policy.ex

lib/pleroma/web/activity_pub/mrf/keyword_policy.ex

...leroma/web/activity_pub/mrf/no_placeholder_text_policy.ex

lib/pleroma/web/activity_pub/mrf/noop_policy.ex

lib/pleroma/web/activity_pub/mrf/normalize_markup.ex

lib/pleroma/web/activity_pub/mrf/reject_non_public.ex

lib/pleroma/web/activity_pub/mrf/simple_policy.ex

lib/pleroma/web/activity_pub/mrf/tag_policy.ex

lib/pleroma/web/activity_pub/mrf/user_allowlist.ex