Document configuring non-primary db with env var

This documents the behaviour added in rails#36773 which allows setting a
database connection URL in an environment variable named
"#{name.upcase}_DATABASE_URL".

Author: floehopper

guides/source/active_record_multiple_databases.md

@@ -86,6 +86,13 @@ production:
     replica: true
 ```
 
+NOTE: In the same way that it's possible to [configure](configuring.html#configuring-a-database)
+the primary database by setting the connection string in the `DATABASE_URL` environment variable,
+it's possible to configure other databases using an environment variable prefixed with the
+configuration key in uppercase. For example, setting:
+`ANIMALS_DATABASE_URL="mysql2://username:password@host/database"` overrides the `animals`
+configuration in the "production" environment.
+
 When using multiple databases, there are a few important settings.
 
 First, the database name for `primary` and `primary_replica` should be the same because they contain

railties/lib/rails/generators/rails/app/templates/config/databases/mysql.yml.tt

@@ -49,6 +49,12 @@ test:
 #   production:
 #     url: <%%= ENV["MY_APP_DATABASE_URL"] %>
 #
+# The connection URL for non-primary databases can also be configured using a
+# similar environment variable prefixed with the configuration key in uppercase.
+# For example:
+#
+#   CACHE_DATABASE_URL="mysql2://cacheuser:cachepass@localhost/cachedatabase"
+#
 # Read https://guides.rubyonrails.org/configuring.html#configuring-a-database
 # for a full overview on how database connection configuration can be specified.
 #

railties/lib/rails/generators/rails/app/templates/config/databases/postgresql.yml.tt

@@ -81,6 +81,12 @@ test:
 #   production:
 #     url: <%%= ENV["MY_APP_DATABASE_URL"] %>
 #
+# The connection URL for non-primary databases can also be configured using a
+# similar environment variable prefixed with the configuration key in uppercase.
+# For example:
+#
+#   CACHE_DATABASE_URL="postgres://cacheuser:cachepass@localhost/cachedatabase"
+#
 # Read https://guides.rubyonrails.org/configuring.html#configuring-a-database
 # for a full overview on how database connection configuration can be specified.
 #

railties/lib/rails/generators/rails/app/templates/config/databases/trilogy.yml.tt

@@ -51,6 +51,12 @@ test:
 #   production:
 #     url: <%%= ENV["MY_APP_DATABASE_URL"] %>
 #
+# The connection URL for non-primary databases can also be configured using a
+# similar environment variable prefixed with the configuration key in uppercase.
+# For example:
+#
+#   CACHE_DATABASE_URL="trilogy://cacheuser:cachepass@localhost/cachedatabase"
+#
 # Read https://guides.rubyonrails.org/configuring.html#configuring-a-database
 # for a full overview on how database connection configuration can be specified.
 #