fix: add a note about ordering (or lack of)

extensions/functions_aggregate_generic.yaml

@@ -39,6 +39,9 @@ aggregate_functions:
     description: >-
       First value from a group of values.
 
+      The definition of "first" depends on ordering. If ordering is not defined, the returned value may be chosen
+      non-deterministically.
+
       If `ignore_nulls` is set to `TRUE`, the function will look for the first non-null value.
       If set to `FALSE`, the function will return the first value it sees, even if null.
 
@@ -58,6 +61,9 @@ aggregate_functions:
     description: >-
       Last value from a group of values.
 
+      The definition of "last" depends on ordering. If ordering is not defined, the returned value may be chosen
+      non-deterministically.
+
       If `ignore_nulls` is set to `TRUE`, the function will look for the first non-null value.
       If set to `FALSE`, the function will return the first value it sees, even if null.