pip: Documentation doesn't explain that we build wheels and install from them

Should the docs clarify the process of installing? Specifically that we run bdist_wheel to build a wheel that we cache, and then we install it without using setup.py.

https://pip.pypa.io/en/latest/reference/pip_install/#build-system-interface hints at the details, by saying “All packages will be attempted to built into wheels”, but it also says “The install command should implement the complete process of installing the package to the target directory XXX”. (Also, we don’t document what happens if the wheel package isn’t available, and TBH I’m not sure I know what we actually do - do we fall back on setup.py install?)

This may not be worth fixing, as this whole area of the docs will need revising for PEP 517, but if that’s the decision, then this issue can remain as a reminder that we need to do that.