OSS workflow: not possible to run `//tests/core/help` so docs PRs need FB internal fixups

[lf-]

This is because tests is in project.ignore.

There's golden tests in there which can't be run outside FB, so they are ~guaranteed to fail for external contributions.

• docs: document how to use fish completion #1013
• Docs: mention where things come from more often #1025

At least one proximate cause is #1026, which is that the pytest macros being used by the buck2 test suite are not available externally.