Apply suggestions from code review

Author: nathanl

lib/phoenix/tracker.ex

@@ -72,12 +72,12 @@ defmodule Phoenix.Tracker do
 
   ## Application Shutdown
 
-  When a tracker shuts down, the other nodes do not assume it has been gone
+  When a tracker shuts down, the other nodes do not assume it is gone
   for good. After all, in a distributed system, it is impossible to know if something
   is just temporarily unavailable or if it has crashed.
 
   For this reason, when you call `System.stop()` or the Erlang VM receives a
-  `SIGTERM` - any presences that the local tracker instance has will continue to
+  `SIGTERM`, any presences that the local tracker instance has will continue to
   be seen as present by other trackers in the cluster until the `:down_period`
   for the instance has passed.
 
@@ -292,7 +292,7 @@ defmodule Phoenix.Tracker do
       (30s down detection). Note: This must be at least 2x the `broadcast_period`.
     * `permdown_on_shutdown` - boolean; whether to immediately call
       `graceful_permdown/1` on the tracker during a graceful shutdown. See
-      'Application Shutdown' section. You can only safely set this point if `Phoenix.Tracker`
+      'Application Shutdown' section. You can only safely set this if `Phoenix.Tracker`
       is mounted at the root of your supervision tree and the strategy is `:one_for_one`.
       Default `false`.
     * `:permdown_period` - The interval in milliseconds to flag a replica