FindPythonInterp: Document instead of warn find_package order. (#13794)

Document in both FindPythonInterp.cmake and FindPythonLibs.cmake that
find_package(PythonInterp) should be called before find_package(PythonLibs).

This prevents warnings when a project does

  find_package(PythonInterp)
  find_package(PythonLibs)

then later another

  find_package(PythonInterp)

Suggested-by: Brad King <brad.king@kitware.com>
Revert topic 'fix-Qt4-moc-command-depends'

It will be revised and restored.
Merge topic 'fix-Qt4-moc-command-depends' into next

b7af338c Revert topic 'fix-Qt4-moc-command-depends'
Merge topic 'different-python-header-libs-exe-0013794' into next

9589c70b FindPythonInterp: Document instead of warn find_package order. (#13794)