Add missing documentation comments

[euanh]

Motivation

All public declarations should have documentation comments, but a number did not. The linter configuration previously accepted these files but now rejects them; the linter depends on the upstream swift container image, which is updated from time to time.

Modifications

Add documentation comments for public declarations identified by the format linter.

Result

All public declarations are documented.

Test Plan

The linter passes locally.
No functional change, all unit tests continue to pass.