Styleguide cleanup part 3.

* Add missing discussion thread links
* Canonicalize existing links
* Standardize pattern for contents of final column: notes, then Google Style Guide link, then discussion thread links
* Move std::nullptr_t from a note in the language features section to its own entry in the library features section
* Remove entry for "containers of move-only types" since AFAICT this isn't really a "feature" with explicit separate documentation, it never had any discussion, and usage of these is commonplace in the codebase now and doesn't need explicit encouragement/clarity
* Updated/clarified some comments

BUG=none
TEST=none

Review-Url: https://codereview.chromium.org/2466103002
Cr-Commit-Position: refs/heads/master@{#429399}