Clarify documented behavior of versioned manifest files in usage docs

[stmontgomery]

This updates and clarifies the Version-specific Manifest Selection section of the usage documentation to clarify the behavior when having either newer or older tools versions.

Motivation:

In general, when using the versioned manifest feature, it's preferable to have the unversioned Package.swift represent the newest-supported tools version, and only use version-specific manifest files for older versions. This provides a better workflow because it allows you to more easily drop support for older versions when ready (by avoiding the need to modify the unversioned file), among other benefits.

Modifications:

• "Reverse" the example shown in this documentation by having the unversioned file be newest.
• Modernize the version numbers shown in examples.
• Fix some non-inclusive language usages.
• Add a footnote with some helpful historical context, and giving a recommendation.

[stmontgomery]

@swift-ci please test

[bkhouri]

@swift-ci test self hosted windows

[dschaefer2]

Thanks Stuart!

[stmontgomery]

@swift-ci smoke test macOS