Cleanup: move public doc-strings into headers for 'python/intern'

Ref T92709

source/blender/python/BPY_extern_python.h

@@ -32,6 +32,8 @@ extern "C" {
 #include <stdio.h>
 
 /* bpy_interface.c */
+
+/** Call #BPY_context_set first. */
 void BPY_python_start(struct bContext *C, int argc, const char **argv);
 void BPY_python_end(void);
 void BPY_python_reset(struct bContext *C);