Update the 5.5.0 news entry and release page.

Specifically:

- Linked to the relevant manual pages for all documented new features, rather
  than RFCs or just having bare list items.

- Linked to php.net rather than www.php.net, per the canonical URIs.

- Made the punctuation consistent.

- Fixed up grammar.

There'll be another commit coming later tonight linking to the documentation
for the couple of items that didn't have documentation prior to 5.5.0 final;
that's waiting on a documentation build to finish and be pushed out.