Document that StringMap iteration order is non-deterministic.

llvm-svn: 155040

llvm/docs/ProgrammersManual.html

@@ -1734,6 +1734,9 @@ already in the table, and each pair in the map is store in a single allocation
 
 <p>StringMap also provides query methods that take byte ranges, so it only ever
 copies a string if a value is inserted into the table.</p>
+
+<p>StringMap iteratation order, however, is not guaranteed to be deterministic,
+so any uses which require that should instead use a std::map.</p>
 </div>
 
 <!-- _______________________________________________________________________ -->