Add implementation note for how to parse room, alias, and user IDs

Room IDs should be treated as fully opaque and not parsed. Aliases and user IDs can be parsed with something like:
```
const parts = id.split(':');
const localpart = parts[0];
const domain = parts.slice(1).join(':');
```

No file or test is named; locate the Matrix specification section defining room, alias, and user IDs. Add an implementation note stating that room IDs are opaque while aliases and user IDs may be split at the first colon, and verify the wording matches the specification.

Written by the indexing model from the issue text.