mgr/vol: add comments to explain queuing data structures

Add some comment to explains the purpose and details of data structures
that are used for queuing of async jobs since right now it's not obvious
from initialization.

Signed-off-by: Rishabh Dave <[email protected]>

Author: rishabh-d-dave

src/pybind/mgr/volumes/fs/async_job.py

@@ -118,10 +118,16 @@ class AsyncJobs(threading.Thread):
     def __init__(self, volume_client, name_pfx, nr_concurrent_jobs):
         threading.Thread.__init__(self, name="{0}.tick".format(name_pfx))
         self.vc = volume_client
-        # queue of volumes for starting async jobs
+        # self.q is a deque of names of a volumes for which async jobs needs
+        # to be started.
         self.q = deque()  # type: deque
-        # volume => job tracking
+
+        # self.jobs is a dictionary where volume name is the key and value is
+        # a tuple containing two members: the async job and an instance of
+        # threading.Thread that performs that job.
+        # in short, self.jobs = {volname: (async_job, thread instance)}.
         self.jobs = {}
+
         # lock, cv for kickstarting jobs
         self.lock = threading.Lock()
         self.cv = threading.Condition(self.lock)