Document meson-python requirements on how `import('python').find_installation()` is called

meson-python invokes meson and meson may be run by a different Python interpreter than the one used to run meson-python via pip or build or other pep517 frontend. meson-python uses a native file to pass the python interpreter to build for to meson. AFAIK, this works only if a path to a Python interpreter is not explicitly passed as an argument to `import('python').find_installation()`. This should be documented.

Start by locating the project documentation that explains meson-python's native file and its use of Meson's import('python').find_installation() entry point. Document the requirement about not passing an explicit Python interpreter path, and consider the work done when users can understand which interpreter meson-python passes to Meson and the limitation involved.

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