document that persistent history is not affected by buffer limits

default.yaml

 
     # options to store history messages in a persistent database (currently only MySQL).
     # in order to enable any of this functionality, you must configure a MySQL server
-    # in the `datastore.mysql` section.
+    # in the `datastore.mysql` section. enabling persistence overrides the history
+    # size limits above (`channel-length`, `client-length`, etc.); persistent
+    # history has no limits other than those imposed by expire-time.
     persistent:
         enabled: false
 

traditional.yaml

 
     # options to store history messages in a persistent database (currently only MySQL).
     # in order to enable any of this functionality, you must configure a MySQL server
-    # in the `datastore.mysql` section.
+    # in the `datastore.mysql` section. enabling persistence overrides the history
+    # size limits above (`channel-length`, `client-length`, etc.); persistent
+    # history has no limits other than those imposed by expire-time.
     persistent:
         enabled: false