Remove unused @params in docs

`tabs` and `options` parameters to `fix_textareas!` have been removed,
see 71bda112041c52b89dce3f626bf38636bb6e2181 and
2efd12f27e3d5e60bcbe1b34a9507518262747e4.
Update links to options in REFERENCE.md

Update links that haven't been updated after change in options handling.
Update :ruby filter docs in REFERENCE.md

Ruby filter no longer writes stdout to the document. Update REFERENCE.md
to reflect this.
Minor formatting / whitespace