Server hooks: explain different GL_PROTOCOL values

What does this MR do?

Explain the GL_PROTOCOL value during custom hook execution.

Conformity

• [-] I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)