Some more work on the unordered containers documentation.

[SVN r2889]

doc/hash_equality.qbk

@@ -0,0 +1,23 @@
+[section:hash_equality Equality Predicates and Hash Functions]
+
+[/TODO: A better introduction to hash functions?]
+
+While the associative containers use an ordering relation to specify how the
+elements are stored, the unordered associative containers use an equality
+predicate and a hash function. For example [classref boost::unordered_set]
+is declared as:
+
+    template<typename Value,
+           typename Hash = ``[classref boost::hash]``<Value>, 
+           typename Pred = std::equal_to<Value>, 
+           typename Alloc = std::allocator<Value> > 
+    class ``[classref boost::unordered_set unordered_set]``;
+
+The hash function comes first as you might want to change the hash function
+but not the equality predicate, while if you were to change the behaviour
+of the equality predicate you would have to change the hash function to match
+it.
+
+For example, if you wanted to use
+
+[endsect]