add clarifications in permissions docs

[donaldchen]

This PR adds the docs clarifications requested from #7318 .

A few notes:

• CREATE_GUILD_EXPRESSIONS and CREATE_EVENTS permissions are still not public
• I unabbreviated T, V, and S in a new table, but I didn't map them to every channel type because the mapping is different based on the permission. For example, T applies to GUILD_VOICE (for its associated text chat) for a permission like SEND_MESSAGES, but it doesn't apply to GUILD_VOICE for a permission like CREATE_PUBLIC_THREADS

[advaith1]

For example, T applies to GUILD_VOICE (for its associated text chat) for a permission like SEND_MESSAGES

I think every permission that applies to text-in-voice has V for voice now?

[donaldchen]

I think every permission that applies to text-in-voice has V for voice now?

Oh I see. In that case, I think I can make a mapping from T to channel types that is consistent for every permission then