Add some comments explaining what these files do

Just because we don't have such comments for other files, doesn't mean we can't
start applying some best practices.

program_logic/ectx_language.v

+(** An axiomatization of evaluation-context based languages, including a proof
+    that this gives rise to a "language" in the Iris sense. *)
 From iris.algebra Require Export base.
 From iris.program_logic Require Export language.
 

program_logic/ectx_weakestpre.v

+(** Some derived lemmas for ectx-based languages *)
 From iris.program_logic Require Export ectx_language weakestpre lifting.
 From iris.program_logic Require Import ownership.