Adjust documentation of permissions

## What does this MR do?
This MR improves the documentation of the permissions by mentioning that internal projects do not enforce the Guest role.
## Are there points in the code the reviewer needs to double check?
No code was touched - only the documentation was adjusted.
## Why was this MR needed?
Without this adjustment it might not be clear how you can set up an internal project to be pullable by everyone whilst having the issue tracker and labels managed by permitted people only.
## What are the relevant issue numbers?
#13679 

See merge request !2925