Updated CHANGELOG.md for post 1.5.1 release & minor cleanup (#785)

* Fix typo in PR reference syntax in unreleased improvement notes in CHANGELOG.md.
* Add note about still extant longstanding completion issues in unreleased improvement notes in CHANGELOG.md.
* Fix typos in CHANGELOG.md.
* Improve punctuation & grammar in CHANGELOG.md.
* Trim trailing spaces in CHANGELOG.md.

Signed-off-by: Ross Goldberg <484615+rgoldberg@users.noreply.github.com>

Author: rgoldberg