document minor backwards-incompatible change in Kwant 1.3

Since this does not seem to justify a new bugfix release of Kwant 1.3,
I apply this commit to the 1.4 stable branch.

doc/source/pre/whatsnew/1.3.rst

@@ -219,3 +219,10 @@ image in the fundamental domain.
 
 This change is documented here for completeness.  We expect that the vast
 majority of users of Kwant will not be affected by it.
+
+ .. _whatsnew13-params-api-change:
+
+API change that affects low-level systems
+-----------------------------------------
+The `~kwant.system.System.hamiltonian` method of low-level systems must now accept a
+`params` keyword parameter.

doc/source/pre/whatsnew/1.4.rst

@@ -343,3 +343,5 @@ Changes in Kwant 1.4.1
   adding the entry `Value functions may no longer take unnamed arguments`_.
 - Kwant no longer requires the existence of a `parameters` attribute for
   low-level systems.
+- A note about an :ref:`whatsnew13-params-api-change` that ocurred in Kwant
+  1.3 was added.