Document the behaviour of ResponseCache

it looks like everything that uses ResponseCache expects to have to
`make_deferred_yieldable` its results. It's debatable whether that is the best
approach, but let's document it for now to avoid further confusion.

synapse/util/caches/response_cache.py

@@ -31,6 +31,18 @@ class ResponseCache(object):
         self.timeout_sec = timeout_ms / 1000.
 
     def get(self, key):
+        """Look up the given key.
+
+        Returns a deferred which doesn't follow the synapse logcontext rules,
+        so you'll probably want to make_deferred_yieldable it.
+
+        Args:
+            key (str):
+
+        Returns:
+            twisted.internet.defer.Deferred|None: None if there is no entry
+            for this key; otherwise a deferred result.
+        """
         result = self.pending_result_cache.get(key)
         if result is not None:
             return result.observe()
@@ -38,6 +50,26 @@ class ResponseCache(object):
             return None
 
     def set(self, key, deferred):
+        """Set the entry for the given key to the given deferred.
+
+        *deferred* should run its callbacks in the sentinel logcontext (ie,
+        you should wrap normal synapse deferreds with
+        logcontext.run_in_background).
+
+        Returns a new Deferred which also doesn't follow the synapse logcontext
+        rules, so you will want to make_deferred_yieldable it
+
+        (TODO: before using this more widely, it might make sense to refactor
+        it and get() so that they do the necessary wrapping rather than having
+        to do it everywhere ResponseCache is used.)
+
+        Args:
+            key (str):
+            deferred (twisted.internet.defer.Deferred):
+
+        Returns:
+            twisted.internet.defer.Deferred
+        """
         result = ObservableDeferred(deferred, consumeErrors=True)
         self.pending_result_cache[key] = result