Describe prune_unread_entries in docstrings (#11876)

Should have been caught in #10826.

changelog.d/11876.misc

@@ -0,0 +1 @@
+Improve internal docstrings in `synapse.util.caches`.

synapse/util/caches/deferred_cache.py

@@ -81,13 +81,14 @@ def __init__(
         Args:
             name: The name of the cache
             max_entries: Maximum amount of entries that the cache will hold
-            keylen: The length of the tuple used as the cache key. Ignored unless
-               `tree` is True.
             tree: Use a TreeCache instead of a dict as the underlying cache type
             iterable: If True, count each item in the cached object as an entry,
                 rather than each cached object
             apply_cache_factor_from_config: Whether cache factors specified in the
                 config file affect `max_entries`
+            prune_unread_entries: If True, cache entries that haven't been read recently
+                will be evicted from the cache in the background. Set to False to
+                opt-out of this behaviour.
         """
         cache_type = TreeCache if tree else dict
 

synapse/util/caches/descriptors.py

@@ -254,9 +254,17 @@ def foo(self, key, cache_context):
             return r1 + r2
 
     Args:
+        orig:
+        max_entries:
         num_args: number of positional arguments (excluding ``self`` and
             ``cache_context``) to use as cache keys. Defaults to all named
             args of the function.
+        tree:
+        cache_context:
+        iterable:
+        prune_unread_entries: If True, cache entries that haven't been read recently
+            will be evicted from the cache in the background. Set to False to opt-out
+            of this behaviour.
     """
 
     def __init__(

synapse/util/caches/lrucache.py

@@ -340,6 +340,12 @@ def __init__(
 
             apply_cache_factor_from_config (bool): If true, `max_size` will be
                 multiplied by a cache factor derived from the homeserver config
+
+            clock:
+
+            prune_unread_entries: If True, cache entries that haven't been read recently
+                will be evicted from the cache in the background. Set to False to
+                opt-out of this behaviour.
         """
         # Default `clock` to something sensible. Note that we rename it to
         # `real_clock` so that mypy doesn't think its still `Optional`.