Add guidelines for formatting errcontext strings

doc/src/sgml/sources.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/sources.sgml,v 2.35 2009/10/08 02:39:16 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/sources.sgml,v 2.36 2010/06/28 17:48:26 petere Exp $ -->
 
  <chapter id="source">
   <title>PostgreSQL Coding Conventions</title>
@@ -444,6 +444,12 @@ Hint:       the addendum
     inappropriate in other languages).
    </para>
 
+   <para>
+    Error context strings: Do not capitalize the first letter and do
+    not end the string with a period.  Context strings should normally
+    not be complete sentences.
+   </para>
+
    <para>
     Rationale: Avoiding punctuation makes it easier for client applications to
     embed the message into a variety of grammatical contexts.  Often, primary