Begin including SPI documentation in local builds, and fix all newly-revealed DocC issues

[stmontgomery]

Begin including documentation for @_spi declarations in local development (non-distribution) builds, and fix all newly-revealed DocC issues in those SPI declarations.

I also took this opportunity to simplify the mechanism for conditionalizing certain package settings for distribution builds, and fix a few other documentation omissions and inconsistencies I noticed.

Checklist:

• [x] Code and documentation should follow the style of the Style Guide.
• [x] If public symbols are renamed or modified, DocC references should be updated.

Resolves rdar://118416838

[stmontgomery]

@swift-ci please test

[grynspan]

Please hold on to this PR.