Fix typo in Rails 4.2 release notes
Merge pull request #17794 from zavan/fix-typo

Fix typo in Rails 4.2 release notes
Fix format "Form Helpers" guide

The `"_tag"` was interpreted as the start of a italics.
Merge pull request #17796 from leafac/fix-format

Fix format "Form Helpers" guide
edits in the release notes for 4.2
removes double mention of deliver_now
rails -> bin/rails in index.html
Allow precision option for MySQL datetimes