Document `source-repository-package` better. E.g., what counts as `tag`?

See https://cabal.readthedocs.io/en/latest/cabal-project.html#specifying-packages-from-remote-version-control-locations for the latest documentation.
https://github.com/haskell/cabal/blob/8e648ae06b8d456a698f105df0eaa3a46bd55e68/doc/cabal-project.rst#L155-L195
I am missing the information here what counts as a `tag`?
- [x] git hash. This is given in the example.
- [ ] git tag. I happen to know this.
- [ ] git branch name? (No, see #6888)
- [ ] github pr??

There are field types listed here, but one cannot click on them to be told their semantics.

Also, the example is a bit impoverished, it could be more diverse, in these dimensions:
- type of vcs (atm. `git` only)
- type of url (atm. `https` only)
- type of tag (atm. hash only)

If there were more examples, one could infer more of the semantics of `tag`, `location` etc.