[ci skip] document `statement_limit`

Per discussion at https://github.com/rails/rails/issues/14645#issuecomment-40499409

guides/source/configuring.md

@@ -646,7 +646,7 @@ development:
   pool: 5
 ```
 
-Prepared Statements can be disabled thus:
+Prepared Statements are enabled by default on PostgreSQL. You can be disable prepared statements by setting `prepared_statements` to `false`:
 
 ```yaml
 production:
@@ -654,6 +654,16 @@ production:
   prepared_statements: false
 ```
 
+If enabled, Active Record will create up to `1000` prepared statements per database connection by default. To modify this behavior you can set `statement_limit` to a different value:
+
+```
+production:
+  adapter: postgresql
+  statement_limit: 200
+```
+
+The more prepared statements in use: the more memory your database will require. If your PostgreSQL database is hitting memory limits, try lowering `statement_limit` or disabling prepared statements.
+
 #### Configuring an SQLite3 Database for JRuby Platform
 
 If you choose to use SQLite3 and are using JRuby, your `config/database.yml` will look a little different. Here's the development section: