Restore DocC @Comment blocks to documentation line comments

[stmontgomery]

This reverts a portion of the changes I made in #836 which converted triple-/// style DocC @Comment blocks to double-// style code comments. That change was needed at the time because our documentation was still rendered using pre-Swift 6.1 toolchains, but now @Comment blocks are supported by DocC in documentation comments (as of swiftlang/swift-docc#1107) so we can restore these.

I needed to move some of these comment blocks up before ## See Also sections to avoid a DocC warning about items in lists not being links.

Checklist:

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

[stmontgomery]

@swift-ci please test