Update information on API in developer guide (#18765)
Summary of the issue:
In practice, there have been exceptions to the NVDA API not documented in the developer guide.
Notably:
Preserving transitive imports: imports may be removed/updated in modules. Developers should always import from the original source module, rather than rely on an import from another module.
pip packages. Add-ons should rely on their own versions of pip packages. One exception to consider is updating wxPython, which in the past we have considered an API breaking change in itself.
