update AP, matrix, ssb

protocols/matrix.md

@@ -4,13 +4,15 @@ Matrix is a protocol for replicating a signed history of JSON objects in realtim
 
 ### Identity
 
-A matrix user ID includes the username along with the user's homeserver. For example:
+A Matrix [identifier](https://matrix.org/docs/spec/appendices#common-identifier-format) takes the form of `*localpart:homeserver`, where \* is a “sigil” character which is used to identify the entity’s type. The sigil character "@" states that the entity is a Matrix user ID, and the "localpart" is an identity allocated by that homeserver. For example:
 
 `@bob:matrix.org`
 
+Other sigil IDs include "!" for Room ID, "$" for Event ID, "+" for Group ID, and "#" for room alias.
+
 User accounts, once created on a homeserver, cannot be migrated. To change servers, a user must make a new account. [Automated tooling](https://modular.im/tools/matrix-migration) exists to help with inviting the new account into rooms the previous account was in.
 
-Users have a Matrix user ID, but can also use 3rd party IDs. Matrix identity servers map 3rd party IDs such as email addresses, social accounts, and phone numbers to Matrix ids. The use of this service is optional. A globally federated cluster of trusted identity servers verify and replicate the mappings, although this is considered a stopgap solution until a fully decentralized identity solution is adopted.
+Users have a Matrix user ID, but can also use [3rd party IDs (3PIDs)](https://matrix.org/docs/spec/appendices#pid-types). Matrix identity servers map 3rd party IDs such as email addresses and phone numbers to Matrix ids. The use of this service is optional. A globally federated cluster of trusted identity servers verify and replicate the mappings, although this is considered a stopgap solution until a fully decentralized identity solution is adopted.
 
 User IDs used in conversations will soon be decoupled from permanent IDs, allowing one to decorrelate users from their messages.