[1.6.x] Fixed #22071 -- Added a warning regarding override_settings and aliasing.

Thanks EvilDMP for the suggestion.

Backport of e65c0ccac0 from master
[1.6.x] Fixed #12670 -- Added a note about permissions of files stored in FILE_UPLOAD_TEMP_DIR.

Thanks simon29 for the suggestion.

Backport of 355572ac56 from master