chore: import upstream snapshot with attribution

testing/generation/command_docs/synopses/create_function.txt

@@ -0,0 +1,19 @@
+CREATE [ OR REPLACE ] FUNCTION
+    $name$ ( [ { [ $routine_argmode$ ] [ $argname$ ] $argtype$ [ { DEFAULT | = } $default_expr$ ] } [ , ... ] ] )
+    [ RETURNS [SETOF] $rettype$
+    | RETURNS TABLE ( { $column_name$ $column_type$ } [ , ... ] ) ]
+    { LANGUAGE $lang_name$
+    | TRANSFORM { FOR TYPE $type_name$ } [ , ... ]
+    | WINDOW
+    | { IMMUTABLE | STABLE | VOLATILE }
+    | [ NOT ] LEAKPROOF
+    | { CALLED ON NULL INPUT | RETURNS NULL ON NULL INPUT | STRICT }
+    | { [ EXTERNAL ] SECURITY INVOKER | [ EXTERNAL ] SECURITY DEFINER }
+    | PARALLEL { UNSAFE | RESTRICTED | SAFE }
+    | COST $execution_cost$
+    | ROWS $result_rows$
+    | SUPPORT $support_function$
+    | SET $configuration_parameter$ { TO $value$ | = $value$ | FROM CURRENT }
+    | AS '$definition$'
+    | AS '$obj_file$', '$link_symbol$'
+    } ... [ $sql_body$ ]