matrix: a few words on room upgrades explicitly

pages/matrix.markdown

@@ -29,6 +29,7 @@ I also have a [txt with a list of all my accounts](/txt/matrix.txt) which [has S
   - [What are state resets?](#what-are-state-resets)
     - [How about DAG splits?](#how-about-dag-splits)
   - [Can I have a non-federated room?](#can-i-have-a-non-federated-room)
+  - [What exactly is room upgrading?](#what-exactly-is-room-upgrading)
   - [What are these idlekicks for inactivity, why are they for?](#what-are-these-idlekicks-for-inactivity-why-are-they-for)
     - [But the relaybots look so ugly](#but-the-relaybots-look-so-ugly)
   - [I am told that I should Matrixify my IRC channel, what does that mean?](#i-am-told-that-i-should-matrixify-my-irc-channel-what-does-that-mean)
@@ -179,6 +180,23 @@ For futher reading about ACL:
 - [Matrix Specification on ACL](https://spec.matrix.org/latest/client-server-api/#server-access-control-lists-acls-for-rooms)
 - [matrix-org/matrix-spec#928: Consider handling server ACLs through event auth rules rather than at the network level #928](https://github.com/matrix-org/matrix-spec/issues/928)
 
+### What exactly is room upgrading?
+
+Room upgrading basically means:
+
+1. Create a new room.
+1. Send an event to old room saying "the room has now moved to new room"
+1. Unless upgraded manually, the client copies some state such as power
+   levels from the old room to the new one.
+
+Manual upgrading means poking the API endpoint manually and thus not copying
+creation event (non-federation state) or power levels. For an example see my
+[matrix-tombstone-room.bash script](https://gitea.blesmrt.net/mikaela/scripts/src/branch/master/bash/matrix-tombstone-room.bash)
+
+See also [Matrix Specification on room versions](https://spec.matrix.org/latest/rooms/)
+or `CTRL-F` this page for `/upgraderoom {{site.matrixLatestRoomVersion}}`
+(Element Web command to perform the upgrade).
+
 ### What are these idlekicks for inactivity, why are they for?
 
 Some Matrix rooms decide to connect their channel to IRC maintaining the same users on both sides, which can be heavy for the IRC network depending on bridge type of which there are three "major" variants: