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

diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index a5b32c903d64fb00f35efc68341acc58720cb826..4b5689f4a01b7fffe2a3e9a0f2e2bf59001fd2da 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>