Update routing of fallback auth in the worker docs. (#7048)

changelog.d/7048.doc

@@ -0,0 +1 @@
+Document that the fallback auth endpoints must be routed to the same worker node as the register endpoints.

docs/workers.md

@@ -273,6 +273,7 @@ Additionally, the following REST endpoints can be handled, but all requests must
 be routed to the same instance:
 
     ^/_matrix/client/(r0|unstable)/register$
+    ^/_matrix/client/(r0|unstable)/auth/.*/fallback/web$
 
 Pagination requests can also be handled, but all requests with the same path
 room must be routed to the same instance. Additionally, care must be taken to