Document import sheningans happing with in-editor Python files

[lordloki]

Blender/UPBGE overrides the import function in Python to first look into the datablocks to find some Python module file.

As it can be confusing, an entry in the documentation should make it clear.

See: https://github.com/UPBGE/upbge/issues/1011#issuecomment-461093314 (comment)

Issue moved from UPBGE/upbge