Clarify differences between `/devices` and `/keys/query`

Both POST /keys/query and GET /devices return a list of devices, so it's not entirely clear what the differences are.

The primary differences are:

• /keys/query returns only those devices which have uploaded encryption keys, whereas /devices returns all devices. This in particular is slightly subtle and the spec makes no reference to it.
• /devices cannot be used to query the device list of another user (it is strictly for the use of the local user to populate a "where am I logged in?" dialogue) whereas /keys/query can be used to get device information for other users

Start by comparing the linked POST /keys/query and GET /devices sections in the Matrix client-server specification. Clarify that /keys/query returns only devices with uploaded encryption keys and can query other users, while /devices returns all devices for the local user. Done when these distinctions are explicit in the specification.