docs: correct view listing cost and say what an expression may use

Two pages said list_materialized_views reads every table's schema and
that SHOW MATERIALIZED VIEWS is cheaper. That is the local
implementation; on a db:// connection the SDK asks the deployment for
its views, as SHOW MATERIALIZED VIEWS does.

The computed-column page now says that a literal must fit the column it
meets, and that a deployment's refresh can fail on an expression reading
a struct field, which the embedded library computes, so an expression
for a deployment should use top-level columns.

docs/web/feature-engineering/add-computed-column.mdx

@@ -69,6 +69,16 @@ declared earlier in the same mapping, but not one declared after it.
 The functions available are the ones the
 [SQL dialect](/sql/functions) documents.
 
+A literal in the expression has to fit the column it meets: `x / 2.0` is
+refused when `x` is an integer column, so write `cast(x as double) / 2.0`.
+
+<Warning>
+On a deployment, keep the expression to top-level columns. A deployment's
+refresh can fail on an expression that reads a field inside a struct, such as
+`stats.magic + stats.strength`, which the embedded library computes. Try a new
+expression on a small table first.
+</Warning>
+
 ## Its inputs become load-bearing {#input-constraints}
 
 <Warning>

docs/web/feature-engineering/materialized-views.mdx

@@ -110,9 +110,10 @@ when a view is created through its REST API, which recent versions no longer
 have. Check with the deployment's administrator which version it runs before
 relying on a source.
 
-`SHOW MATERIALIZED VIEWS` lists them, which is cheaper than
-`list_materialized_views` -- that method finds views by reading every table's
-schema, so it costs one open per table.
+`list_materialized_views` asks the deployment for its views, as
+[`SHOW MATERIALIZED VIEWS`](/sql/ddl/materialized-view#show) does. On a local
+database the same method opens each table to read its schema, so it costs one
+open per table.
 
 ## Working with the view {#working-with-the-view}
 

docs/web/sql/ddl/materialized-view.mdx

@@ -36,8 +36,8 @@ SHOW MATERIALIZED VIEWS
 ```
 
 <Note>
-Cheaper than the SDK equivalent. `list_materialized_views` finds views by
-reading every table's schema, costing one open per table; this asks the server.
+This asks the server for its views, as the SDK's `list_materialized_views` does
+on a `db://` connection.
 </Note>
 
 ## Refreshing one {#refresh}