Document that StringMap iteration order is non-deterministic.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@155040 91177308-0d34-0410-b5e6-96231b3b80d8

Author: Jim Grosbach

docs/ProgrammersManual.html

@@ -1734,6 +1734,9 @@ <h4>
 
 <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>
 
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