chore: import upstream snapshot with attribution

testing/generation/command_docs/synopses/create_index.txt

@@ -0,0 +1,7 @@
+CREATE [ UNIQUE ] INDEX [ CONCURRENTLY ] [ [ IF NOT EXISTS ] $name$ ] ON [ ONLY ] $table_name$ [ USING $method$ ]
+    ( { { $column_name$ | ( $expression$ ) } [ COLLATE $collation$ ] [ $opclass$ [ ( { $opclass_parameter$ = $value$ } [ , ... ] ) ] ] [ ASC | DESC ] [ NULLS { FIRST | LAST } ] } [ , ... ] )
+    [ INCLUDE ( $column_name$ [ , ... ] ) ]
+    [ NULLS [ NOT ] DISTINCT ]
+    [ WITH ( { $storage_parameter$ [ = $value$ ] } [ , ... ] ) ]
+    [ TABLESPACE $tablespace_name$ ]
+    [ WHERE $predicate$ ]