doc: add much-needed accuracy to pip installation instructions [skip ci]

- mention installing from local sources, not PyPI
- warn against --user installs, which too often screw up users that then
  cannot install projects because ~/.local won't be in sudo's PYTHONPATH
- advise installing with sudo -- current versions of pip assume --user
  for you rather than failing with permission errors, which is great
  unless, like meson, there are compelling reasons to need to install as
  root

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