rabbit_disk_monitor: Document NaN alarm behavior

When disk information is unavailable, the 'available' field in mount
records is set to 'NaN'. Due to Erlang term ordering, 'NaN' < Limit
evaluates to true, which triggers alarms for those mounts.

This is correct fail-safe behavior - when we cannot determine available
disk space, we block publishing to prevent potential disk exhaustion.

Add comments to alarmed_mounts/1 and alarmed_queue_types/1 explaining
this intentional behavior.

Author: lukebakken

deps/rabbit/src/rabbit_disk_monitor.erl

@@ -385,6 +385,8 @@ alarmed_mounts(Mounts) ->
     maps:fold(
       fun (Path, #mount{available = Available,
                         limit = Limit}, Acc) when Available < Limit ->
+              %% Note: 'NaN' < Limit is true (Erlang term ordering), which is
+              %% correct fail-safe behavior when disk info is unavailable
               sets:add_element(Path, Acc);
           (_Path, _Mount, Acc) ->
               Acc
@@ -397,6 +399,8 @@ alarmed_queue_types(MountPoints) ->
       fun (_Path, #mount{available = Available,
                          limit = Limit,
                          queue_types = QTs}, Acc) when Available < Limit ->
+              %% Note: 'NaN' < Limit is true (Erlang term ordering), which is
+              %% correct fail-safe behavior when disk info is unavailable
               sets:union(QTs, Acc);
           (_Path, _Mount, Acc) ->
               Acc