Improve clarity of systemd service file comments

The phrasing seemed to indicate that the options specified in
ExecStart= could not be specified in the config file, necessitating
their inclusion in the service file. However, the options in the
config file simply get overridden by any specified in ExecStart=.

contrib/init/bitcoind.service

@@ -5,8 +5,9 @@
 # See "man systemd.service" for details.
 
 # Note that almost all daemon options could be specified in
-# /etc/bitcoin/bitcoin.conf, except for those explicitly specified as arguments
-# in ExecStart=
+# /etc/bitcoin/bitcoin.conf, but keep in mind those explicitly
+# specified as arguments in ExecStart= will override those in the
+# config file.
 
 [Unit]
 Description=Bitcoin daemon