Update system-info macOS user agent

Add CPU temperature to `status-info`, conditionally display battery
charge percentage, and documents the commands.

system-info/system-info-macOS.sh

@@ -19,9 +19,10 @@ ioreg -w0 -l|grep BatteryInstalled &> /dev/null && \
 while true; do
   # Get the current CPU temperature.
   cpu_temp="`/usr/local/bin/osx-cpu-temp`"
-  # Parse the current battery charge percentage.
-  $has_battery && \
+  if $has_battery; then
+    # Parse the current battery charge percentage.
     battery=" `pmset -g batt | grep --color=never -Eo '\d+%'` ↯"
+  fi
   # Write to the cache file.
   echo "$cpu_temp$battery" > $cache_file
   # Don't spin, sleep instead.