[go_router] Add documentation to some methods in matching.dart

[ValentinVignal]

I was investigating how to fix https://github.com/flutter/flutter/issues/113032 (I will soon public a design proposition as I think the entire RouteMatchList structure needs to be changed from a list to a tree). It is a bit hard to understand what each method was supposed to do so I added some documentation to help future contributors.

Pre-launch Checklist

• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [x] I read the Tree Hygiene wiki page, which explains my responsibilities.
• [x] I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• [x] I signed the CLA.
• [x] The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• [x] I listed at least one issue that this PR fixes in the description above.
• [x] I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• [x] I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] I added new tests to check the change I am making, or this PR is test-exempt.
• [x] All existing and new tests are passing.

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[ValentinVignal]

I would think this is a version exempt ?

[ValentinVignal]

I would think this is a version exempt ?

Should the label "override: no versioning needed" be added to this PR ? Or should I create a new version?

[chunhtai]

cc @johnpryan for secondary review