Add a note about wrapper scripts and sockets

Any wrapper scripts which `exec`, or other indirections in
`ExecStart`, may result in activated socket file descriptors being closed
before they reach the puma master process. For example, if using `bundle exec`,
pass the `--keep-file-descriptors` flag.

`bundle exec` can be avoided by using a `puma` executable generated by
`bundle binstubs puma`.

This commit also remove a couple of trailing whitespaces

Ref: #1499

docs/systemd.md

@@ -71,6 +71,13 @@ listening sockets open across puma restarts and achieves graceful
 restarts, including when upgraded puma, and is compatible with both
 clustered mode and application preload.
 
+**Note:** Any wrapper scripts which `exec`, or other indirections in
+`ExecStart`, may result in activated socket file descriptors being closed
+before they reach the puma master process. For example, if using `bundle exec`,
+pass the `--keep-file-descriptors` flag. `bundle exec` can be avoided by using a
+`puma` executable generated by `bundle binstubs puma`. This is tracked in
+[#1499].
+
 **Note:** Socket activation doesn't currently work on jruby. This is
 tracked in [#1367].
 
@@ -254,8 +261,8 @@ Restart=no
 
 # `puma_ctl restart` wouldn't work without this. It's because `pumactl`
 # changes PID on restart and systemd stops the service afterwards
-# because of the PID change. This option prevents stopping after PID 
+# because of the PID change. This option prevents stopping after PID
-# change.  
+# change.
 RemainAfterExit=yes
 
 [Install]