Document that chr(0) is not supported, and why.

diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 6306edb27b4ab6d8e7df8111e8f83244faf45275..ac9ee81837e595985ce76b13cf90e5396d350e0d 100644 (file)
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -1340,7 +1340,8 @@
         Character with the given code. For <acronym>UTF8</acronym> the
         argument is treated as a Unicode code point. For other multibyte
         encodings the argument must designate a strictly
-        <acronym>ASCII</acronym> character.
+        <acronym>ASCII</acronym> character.  The NULL (0) character is not
+        allowed because text data types cannot reliably store such bytes.
        </entry>
        <entry><literal>chr(65)</literal></entry>
        <entry><literal>A</literal></entry>