Copy-edit the MessageVerifier documentation [ci skip]
Fix MessageVerifier's #verified in CHANGELOG

[ci skip]

As confirmed by @lleger (the author of `verified`) [in this comment](https://github.com/rails/rails/pull/17727#issuecomment-65488743):

> Actually, it no longer returns false explicitly (bc8cc56), so I guess the CHANGELOG isn't totally accurate. It returns nil instead (but the functionality isn't practically different).