Give clearer example of multiple query predicates in one ql file

The new names aren't great, so feel free to change them, but I think we do need an explicit example of updating two relations using one ql file.

Author: owen-mc

docs/prepare-db-upgrade.md

@@ -59,14 +59,16 @@ extended.rel: reorder input.rel (int id, string name, int parent) id name parent
 // QLL library, and will run in the context of the *old* dbscheme.
 relationname.rel: run relationname.qlo
 
-// Create relationname.rel by running the query predicate 'predicatename' in
-// relationname.qlo and writing the query results as a .rel file. This command
+// Create relation1.rel by running the query predicate 'predicate1' in upgrade.qlo
+// and writing the query results as a .rel file, and running 'predicate2' in
+// upgrade.qlo and writing the query results as a .rel file. This command
 // expects the upgrade relation to be a query predicate, which has the advantage
 // of allowing multiple upgrade relations to appear in the same .ql file as
-// multiple query predicates. The query file should be named relationname.ql and
+// multiple query predicates. The query file should be named upgrade.ql and
 // should be placed in the upgrade directory. It should avoid using the default
 // QLL library, and will run in the context of the *old* dbscheme.
-relationname.rel: run relationname.qlo predicatename
+relation1.rel: run upgrade.qlo predicate1
+relation2.rel: run upgrade.qlo predicate2
 ```
 
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