[4.0] Standard library documentation revisions

[natecook1000]

This is a cherry pick of #11249 and #11273.

• Explanation: Includes documentation revisions to several parts of the standard library, including the key path appending methods, the new String index, corrections to some Codable API documentation, and work to make sure concrete numeric types have the expected documentation.
• Scope: This includes only changes to the standard library's inline documentation, there is no functional change.
• Radar/SR: rdar://problem/33701894
• Risk: Very Low
• Testing: The unit and validation test suites.

[natecook1000]

@swift-ci Please test