address review comments on dag cache docs and add improvement newsfragment

Author: aeroyorch

airflow-core/docs/faq.rst

@@ -728,7 +728,7 @@ in memory. Configure this in the ``[api]`` section:
     dag_cache_size = 64    ; max cached versions (0 = no size cap, TTL still applies)
     dag_cache_ttl = 3600   ; seconds before a cached entry expires (0 = LRU only)
 
-Setting both to 0 disables eviction entirely, matching pre-3.2 behavior.
+Setting both to 0 disables eviction entirely, matching pre-3.3 behavior.
 
 The cache is keyed by Dag version ID. After a Dag is updated, the API server may serve the
 previous version until the cached entry expires (controlled by ``dag_cache_ttl``).

airflow-core/newsfragments/69774.improvement.rst

@@ -0,0 +1 @@
+``[api] dag_cache_ttl`` is now honored when ``[api] dag_cache_size = 0``, giving the API server TTL-based eviction of cached serialized Dags with no size cap. Previously ``dag_cache_size = 0`` disabled eviction entirely and silently ignored ``dag_cache_ttl``. To keep the previous fully-unbounded behavior, set ``dag_cache_ttl = 0`` as well.

airflow-core/src/airflow/config_templates/config.yml

@@ -1705,7 +1705,10 @@ api:
       description: |
         Size of the LRU cache for SerializedDAG objects in the API server.
         Set to 0 to remove the size cap. Cached entries are then evicted only by
-        ``dag_cache_ttl``, or never if that is also 0 (matching pre-3.2 behavior).
+        ``dag_cache_ttl``, or never if that is also 0 (matching pre-3.3 behavior).
+        Note that the TTL is then not a ceiling on memory. Entries that keep being
+        requested are refreshed and never expire, so memory usage follows the number
+        of distinct Dag versions in active use.
         The cache is keyed by Dag version ID, so lookups by Dag ID
         (e.g., viewing a Dag's details) always query the database for the latest
         version, but the deserialized result is cached for subsequent