Explain what a room ID is

### Your use case

#### What would you like to do?

Provide some explanation to untrained users about what a room ID is and why they should probably be using a room alias if they are trying to share the room with another human.

#### Why would you like to do it?

There tends to be quite a bit of confusion with room IDs because they look like aliases. To a new user it looks like a room ID is exactly what they want, an identifier for the room with a domain on the end. It looks like something that would allow others to join it. Room IDs are not routable though and cannot be used to reliably join a room without additional information. Room IDs also appear to indicate that the domain in the room ID is the "owner" of the room which is untrue.

Room IDs are only useful for software, not for humans.

#### How would you like to achieve it?

A sentence or two should be added in the advanced section of room settings which explains this

### Have you considered any alternatives?

When Room IDs become hashes in the future (with no visible domain) then this is unnecessary

### Additional context

https://github.com/matrix-org/matrix-react-sdk/pull/7600 is a PR making it easier to copy the room ID. This is great for power users or server admins who need to copy it into another program but also could slightly increase the number of untrained users using room IDs.

https://github.com/vector-im/element-meta/issues/1592, https://github.com/vector-im/element-web/issues/18056 are issues requesting that room IDs be linkified in the timeline (which I oppose).

https://github.com/matrix-org/matrix-react-sdk/pull/6048 is my PR which improves the error message when attempting to join a room ID with no via servers.

Here is an example of a server admin getting confused and thinking that because their server's domain is in the room ID, that means they control the room https://matrix.to/#/!ehXvUhWNASUkSLvAGP:matrix.org/$htJmba92wLTP9AoFg4eEWi9IXpgwvXr6G9Sa-kBsNNs?via=matrix.org&via=matrix.breakpointingbad.com&via=libera.chat
> i dont understand what that means. If the room id is tied to my homeserver then i should be able to administrate it

Start in the advanced section of the room settings UI and locate the existing explanatory copy. Add one or two sentences distinguishing room IDs from room aliases, including that IDs are not reliably routable and do not indicate ownership; done means an untrained user can tell which identifier to share.

Written by the indexing model from the issue text.