Restore documentation about multiple constraints.

[ci skip]

Cabal/doc/nix-local-build.rst

@@ -620,6 +620,16 @@ The following settings control the behavior of the dependency solver:
     and other properties a solver can pick for a
     package. For example:
                
+    ::
+
+        constraints: bar == 2.1
+
+    A package can be specified multiple times in ``constraints``, in
+    which case the specified constraints are intersected. This is
+    useful, since the syntax does not allow you to specify multiple
+    constraints at once. For example, to specify both version bounds and
+    flag assignments, you would write:
+
     ::
 
         constraints: bar == 2.1,