Fixes for "External libraries" release notes section

Fixes for "External libraries" release notes section

* Changed the list of libraries to reflect the current states of
  resources/lib and composer.json. (I assumed oojs/oojs-ui refers
  to the PHP library, not the JS libraries. Also, some JS libraries
  may still be missing.)
* Renamed "External libraries" to "External library changes in 1.2X"
  and promoted it to heading level 3 for consistency with the other
  sections and for a sane TOC within the web installer.
* Made additional minor formatting changes.

Change-Id: I195dd1cf1aca1112945dc630e2bb7d8584bb4bb2