Documentation: Explain use of message categories, especially info vs neutral

### Describe the bug

There is no documentation on how message categories in `p->add_msg_if_player()` should be used.

### Steps To Reproduce

n/a

### Expected behavior

Most are self-explanatory but it would be great if they could still be explained somewhere. In particular I found it hard to distinguish between `m_info` and `m_neutral` and I think there should be some official guidance about that somewhere.

### Screenshots

_No response_

### Versions and configuration

n/a

### Additional context

_No response_

Start by locating p->add_msg_if_player() and the m_info and m_neutral category definitions or existing references. Add documentation explaining how message categories should be used, with explicit guidance distinguishing info from neutral; done means the guidance is available in an appropriate project documentation location.

Written by the indexing model from the issue text.