Improve SQL routine docs

docs/src/main/sphinx/routines/function.md

@@ -5,7 +5,7 @@
 ```text
 FUNCTION name ( [ parameter_name data_type [, ...] ] )
   RETURNS type
-  [ LANGUAGE langauge]
+  [ LANGUAGE language]
   [ NOT? DETERMINISTIC ]
   [ RETURNS NULL ON NULL INPUT ]
   [ CALLED ON NULL INPUT ]

docs/src/main/sphinx/routines/introduction.md

@@ -125,11 +125,11 @@ before the following keywords:
 * `REPEAT`
 * `WHILE`
 
-The label is used to name the block in order to continue processing with the
-`ITERATE` statement or exit the block with the `LEAVE` statement. This flow
-control is supported for nested blocks, allowing to continue or exit an outer
-block, not just the innermost block. For example, the following snippet uses the
-label `top` to name the complete block from `REPEAT` to `END REPEAT`:
+The label is used to name the block to continue processing with the `ITERATE`
+statement or exit the block with the `LEAVE` statement. This flow control is
+supported for nested blocks, allowing to continue or exit an outer block, not
+just the innermost block. For example, the following snippet uses the label
+`top` to name the complete block from `REPEAT` to `END REPEAT`:
 
 ```sql
 top: REPEAT
@@ -171,7 +171,12 @@ when writing and running SQL routines:
 
 The following limitations apply to SQL routines.
 
-* Routines must be declared before than can be referenced.
+* Routines must be declared before they are referenced.
 * Recursion cannot be declared or processed.
 * Mutual recursion can not be declared or processed.
 * Queries cannot be processed in a routine.
+
+Specifically this means that routines can not use `SELECT` queries to retrieve
+data or any other queries to process data within the routine. Instead queries
+can use routines to process data. Routines only work on data provided as input
+values and only provide output data from the `RETURN` statement.