Clarify specs for external installer interactions

[ncoghlan]

Closes #1210
Closes #1211
Closes #1212

[sinoroc]

Maybe the page about virtual environments should link to "How venvs work" from the Python documentation. I had understood that in general we should avoid pointing people to PEPs whenever possible (PEPs are a process, not the spec). Which is why I created this PR #1213 (tangentially).

[ncoghlan]

@sinoroc Hmm, that's a good point. I was treating it like one of the other packaging specs, where the PEP was written to be the spec, but that isn't really the case with PEP 405: since it was adding a new standard library module, it put the behaviour spec in the library reference rather than leaving it solely in the PEP. I'll put an unresolved conversation on that so we don't merge this until I've had a chance to reconsider how to structure that bit.

[ncoghlan]

Tweaked the wording of the virtual environments section a bit, but still ended up having to relying primarily on the PEP 405 reference (the venv docs are more "how this module works", and less about the underlying interoperability requirements that PEP 405 laid out in terms of Python runtimes loading pyvenv.cfg files)