gh-157384: Document and test PyImport_CreateModuleFromInitfunc() semantics (GH-157389)

Document that single-phase init modules are added to sys.modules by
the call itself (unlike multi-phase init modules), and that the spec
name identifies the module, so a later call with the same name and a
different init function reuses the cached module without calling the
new init function.  Add regression tests for both behaviors.


Co-authored-by: Petr Viktorin <encukou@gmail.com>