Clarify differences between /devices and /keys/query

[richvdh]

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

[clokep]

Similarly on the S-S side there is GET /user/devices/{userId} and POST /user/keys/query.

@dkasak had found an old reference to this: matrix-org/matrix-spec-proposals#1576 (comment)