Warn about reject-unconstrained

Author: philderbeast

doc/how-to-freeze-versions.rst

@@ -237,6 +237,11 @@ Ensuring everything is frozen
     able to find a different version for any of them, the ``.freeze`` file
     cannot change and, at that point when every dependency is frozen, ``cabal
     freeze`` becomes an idempotent operation.
+    
+.. Warning::
+    Setting :cfg-field:`reject-unconstrained-dependencies` to ``all`` will
+    ensure that dependencies have version constraints but does not require these
+    be equality constraints.
 
 Adding a dependency to one of the packages in a project without freezing harder
 leaves the newly added dependency susceptible to getting updated unexpectedly