SDK / exoplayer

* Move cherrypicked changes from dev-v2 to 2.13.0
* Use a bulleted list instead of comma-separated.
* Standardise everything into present, imperative sentences.
* Remove a couple of mixed-font words (I left cases with a separating
  apostrophe, as these seemed visually clearer).
* Merge multiple issue links into a single set of parentheses.

#minor-release

PiperOrigin-RevId: 355180143