Document grammar for device IDs

The spec is currently missing any definition of the grammar for a device ID. While it is implied that they should just be a random string of characters, at least the following should be explicit:

* Minimum and maximum character (or byte) lengths
* Allowed and disallowed characters
* Any expected patterns, such as starting with a letter followed by N digits

There needs to be some UX consideration beyond considering them opaque byte sequences, since they are displayed visually to users in some cases.

Though not specific to the grammar of the ID, the spec should also address what the scope of uniqueness is for device IDs. Do they need to be universally unique? Or only unique to the user the device belongs to? Or unique to the user's homeserver?

This may be considered part of matrix-org/matrix-spec#174.

Start by locating the existing device ID references in the Matrix specification and review matrix-org/matrix-spec#174 for related context. Determine the grammar requirements, visual UX constraints, and uniqueness scope that the specification must make explicit. Done means the specification documents these decisions consistently.

Written by the indexing model from the issue text.