Escape double dashes in Doxygen input to preserve them

By default double dashes are converted to en-dashes in the output (and triple
ones -- to em-dashes), but this is undesirable when double dashes are used not
as a punctuation mark but in command line options or as C++ decrement
operator, so escape them to avoid such conversion in this case.

interface/wx/hashset.h

@@ -131,7 +131,7 @@
     An iterator is similar to a pointer, and so you can use the usual pointer
     operations: ++it ( and it++ ) to move to the next element, *it to access the
     element pointed to, *it to access the value of the element pointed to.
-    Hash sets provide forward only iterators, this means that you can't use --it,
+    Hash sets provide forward only iterators, this means that you can't use \--it,
     it + 3, it1 - it2.
 
     @library{wxbase}