Improve documentation briefs and misc. details

[eramongodb]

Miscellaneous improvements to API documentation following #1309. Primarily targets cleanup of the brief descriptions displayed on the "Class List" page.

Note

Doxygen seems to fail to generate a class description for a member of a using-declared class. Filed doxygen/doxygen#11326.

The current brief descriptions on the Class List page are not consistently "brief", often unnecessarily including parts of what should be a detailed description instead:

The changes in this PR update all brief descriptions to be more consistent in their format and be properly "brief" in their length:

When able, brief descriptions of auxiliary class types (e.g. "options") simply refer to their primary interface with which they are meant to be used to avoid redundancy and verbosity (e.g. "Used by X"). Redundant leads (e.g. "Class representing...", "Class used to...", etc.) are omitted to improve brevity of descriptions.

Some notable changes that go beyond brief description updates include:

• Adding or updating references to the MongoDB Manual, C++ Driver Manual, and the MongoDB (Drivers) Specification (note: ReadTheDocs; can revert to GitHub file references instead if preferable).
• Add MONGOCXX_DEPRECATED + @deprecated to authentication_exception, which is not used anywhere.
• Notes disambiguating interfaces pertaining to MongoDB server indexes vs. Atlas Search Indexes.
• Add @important to mongocxx::v_noabi::pool documenting it does not conform to the CMAP specification.
• Deferred bulk of read/write concern descriptions (mostly outdated references to pre-3.6 behavior) to the MongoDB Manual.
• Add @deprecated to mongocxx::v_noabi::validation_criteria given there is mostly unused + its primary interfaces (conversion to BSON) are already deprecated as well.

[kevinAlbs]

LGTM. I like the briefer "brief" sections.

note: ReadTheDocs; can revert to GitHub file references instead if preferable

No strong preference. The rendered ReadTheDocs pages seem easier to navigate, but do not appear to have a version history or refer to the Github repo.

[eramongodb]

The rendered ReadTheDocs pages seem easier to navigate, but do not appear to have a version history or refer to the Github repo.

I felt comfortable linking to the ReadTheDocs pages since most of the original references lacked pinning to specific commit versions, blindly tracking the latest commit. I suspect it is sufficient to link to the related page so the relevant content (in whatever position/section they happen to be in at the time) can be found as needed.