DOC: minor tweak to docs on invoking pip/build

[rgommers]

Addresses a comment on gh-704, where a user got tripped up by python -m build building an sdist first (and that's typically not what you want).

[dnicolodi]

We have several other example python -m build invocation in the documentation? Should we change them to python -m build --wheel or is this the time to add that "Creating a binary distribution" we have been talking about and have a note there about the pypa/build unexpected behavior?

[rgommers]

I think we should add --wheel to all other instances of python -m build. The default is useful only for a single CI job or a release of a pure Python package; I can't think of any time I regularly did not add --wheel or -wnx.