docs: git context in rover help

[EverlastingBugstopper]

As a tip, if you don't want to add those variables as parameters you could maybe add some docs for the --help on rover, instead?

Originally posted by @nathanmarcos in https://github.com/apollographql/rover/issues/490#issuecomment-830278913

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This seems like something we could add that might help out users - seems like a good candidate for a DISCUSSION section in any commands that auto-detect GitContext/use the environment variables.

[setchy]

I'd +1 this.

In fact, I raised this issue in the apollo repo just now. There is some stray content in there that provides an 'example' of thee rover cli supports the --branch, --author and --commitId flags

Would be nice if rover either:

• supported the 4 git context environment variables via a rover command line options
• printed out some information re: these 4 environment variables when using the --help flag