Pass period to `BBP_Digest_Event::mail()` hooks.

git-svn-id: https://plugins.svn.wordpress.org/bbpress-digest/trunk@821872 b8457f37-d9ea-0310-8a92-e5e31aec5664
Add documentation missed in [821872]. Also fixes wrong parameter type it introduced.

git-svn-id: https://plugins.svn.wordpress.org/bbpress-digest/trunk@821874 b8457f37-d9ea-0310-8a92-e5e31aec5664
Fix spelling error in documentation for `BBP_Digest_Event::do_event()`

git-svn-id: https://plugins.svn.wordpress.org/bbpress-digest/trunk@821875 b8457f37-d9ea-0310-8a92-e5e31aec5664