Clarify documented behavior of versioned manifest files in usage docs (#8462)

This updates and clarifies the [Version-specific Manifest
Selection](https://github.com/swiftlang/swift-package-manager/blob/main/Documentation/Usage.md#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.

Author: stmontgomery

Documentation/Usage.md

@@ -497,7 +497,7 @@ This feature is intended for use in the following scenarios:
 
 It is *not* expected that the packages would ever use this feature unless absolutely
 necessary to support existing clients. Specifically, packages *should not*
-adopt this syntax for tagging versions supporting the _latest GM_ Swift
+adopt this syntax for tagging versions supporting the _latest released_ Swift
 version.
 
 The package manager supports looking for any of the following marked tags, in
@@ -511,7 +511,7 @@ order of preference:
 
 The package manager will additionally look for a version-specific marked
 manifest version when loading the particular version of a package, by searching
-for a manifest in the form of `Package@swift-3.swift`. The set of markers
+for a manifest in the form of `Package@swift-6.swift`. The set of markers
 looked for is the same as for version-specific tag selection.
 
 This feature is intended for use in cases where a package wishes to maintain
@@ -521,20 +521,28 @@ changes in the manifest API).
 
 It is *not* expected the packages would ever use this feature unless absolutely
 necessary to support existing clients. Specifically, packages *should not*
-adopt this syntax for tagging versions supporting the _latest GM_ Swift
+adopt this syntax for tagging versions supporting the _latest released_ Swift
 version.
 
 In case the current Swift version doesn't match any version-specific manifest,
 the package manager will pick the manifest with the most compatible tools
 version. For example, if there are three manifests:
 
-`Package.swift` (tools version 3.0)
-`Package@swift-4.swift` (tools version 4.0)
-`Package@swift-4.2.swift` (tools version 4.2)
+- `Package.swift` (tools version 6.0)
+- `Package@swift-5.10.swift` (tools version 5.10)
+- `Package@swift-5.9.swift` (tools version 5.9)
 
-The package manager will pick `Package.swift` on Swift 3, `[email protected]` on
-Swift 4, and `[email protected]` on Swift 4.2 and above because its tools
-version will be most compatible with future version of the package manager.
+The package manager will pick `Package.swift` on Swift 6 and above, because its
+tools version will be most compatible with future version of the package manager.
+When using Swift 5.10, it will pick `[email protected]`. Otherwise, when
+using Swift 5.9 it will pick `[email protected]`, and this is the minimum
+tools version this package may be used with.
+
+A package may have versioned manifest files which specify newer tools versions
+than its unversioned `Package.swift` file[^1]. In this scenario, the manifest
+corresponding to the newest-compatible tools version will be used.
+
+[^1]: Support for having a versioned manifest file with a _newer_ tools version was required when the feature was first introduced, because prior versions of the package manager were not aware of the concept and only knew to look for the unversioned `Package.swift`. This is still supported, but there have been many Swift releases since the feature was introduced and it is now considered best practice to have `Package.swift` declare the newest-supported tools version and for versioned manifest files to only specifer older versions.
 
 ## Editing a Package