Document the behavior of Spark string functions for invalid UTF-8 inp…

…ut (facebookincubator#10682)

Summary: Pull Request resolved: facebookincubator#10682

Reviewed By: pedroerp

Differential Revision: D61135197

Pulled By: xiaoxmeng

fbshipit-source-id: 85ab3e13a08628d73b7d14cf4056f8223ebc823d

velox/docs/functions/spark/string.rst

@@ -2,7 +2,12 @@
 String Functions
 ====================================
 
-Unless specified otherwise, all functions return NULL if at least one of the arguments is NULL.
+.. note::
+
+    Unless specified otherwise, all functions return NULL if at least one of the arguments is NULL.
+
+    These functions assume that input strings contain valid UTF-8 encoded Unicode code points.
+    The behavior is undefined if they are not.
 
 .. spark:function:: ascii(string) -> integer