add docs and changelog entry

CHANGELOG.md

@@ -135,6 +135,10 @@ With this release, we dropped support for Coq 8.9.
   threads instead of only a single thread. The derived adequacy lemmas
   are unchanged.
 
+**Changes in `heap_lang`:**
+
+* `wp_pures` now turns goals of the form `WP v {{ Φ }}` into `Φ v`.
+
 The following `sed` script helps adjust your code to the renaming (on macOS,
 replace `sed` by `gsed`, installed via e.g. `brew install gnu-sed`).
 Note that the script is not idempotent, do not run it twice.

docs/heap_lang.md

@@ -60,6 +60,8 @@ Tactics to take one or more pure program steps:
   well as unary and binary arithmetic operators.
 - `wp_pures`: Perform as many pure reduction steps as possible. This
   tactic will **not** reduce lambdas/recs that are hidden behind a definition.
+  If the computation reaches a value, the `WP` will be entirely removed and the
+  postcondition becomes the new goal.
 - `wp_rec`, `wp_lam`: Perform a beta reduction.  Unlike `wp_pure`, this will
   also reduce lambdas that are hidden behind a definition.
 - `wp_let`, `wp_seq`: Reduce a let-binding or a sequential composition.