more detail in docs

models/intermediate/cross/schema.yml

@@ -5,10 +5,13 @@ models:
       granularity is daily. Each record holds a currency pair for a specific
       day, source and version.
 
-      Actual rates are source from xe.com data. The `guessed` and `forecast`
+      Actual rates are sourced from xe.com data. The `guessed` and `forecast`
       versions are built by simply 'pushing'  the first/last exchange rate on
       record. Basically, wherever we dont' have data for a date, we pick the
-      closest actual data point that comes from xe.com.
+      closest actual data point that comes from xe.com. Bear in mind this means
+      that `forecast` version records will change on a daily basis as actual
+      data moves forwards, meaning you shouldn't assume your money amounts
+      converted in the future should always stay put.
 
       Note that, given the dimensionality, getting a simple time series for a
       currency pair will require a bit of filtering.
@@ -68,12 +71,12 @@ models:
           nature). Note that one currency pair can have multiple rate versions
           on the same date.
         tests:
-          - not_null
           - accepted_values:
               values:
                 - guess
                 - actual
                 - forecast
+          - not_null
       - name: updated_at_utc
         data_type: timestamp with time zone
         description: For external sources, this will be the point in time when the