fan: Minor updates to tachometer handling

Add new fields to Command_Templates.md.

Remove unused self.rpm variable.

Use an explicit get_frequency() method in keeping with Klipper's
convention of not "peeking into member variables".

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/extras/pulse_counter.py

@@ -74,6 +74,5 @@ class FrequencyCounter:
                 self._callback(time, self._freq)
         self._last_count = count
 
-    @property
-    def frequency(self):
+    def get_frequency(self):
         return self._freq