Fix missing documentation for many clock faces:

* Move from .c to .h as needed for consistency.
* When missing from both, copy from pull request or wiki.
* When missing entirely, infer functionality from source code.

movement/watch_faces/complication/tachymeter_face.h

@@ -25,6 +25,49 @@
 #ifndef TACHYMETER_FACE_H_
 #define TACHYMETER_FACE_H_
 
+/*
+ * TACHYMETER face
+ *
+ * The Tachymeter complication emulates the tachymeter function often
+ * present in watches, that computes the average speed in [units per hour]
+ * for a given distance given in [units].
+ *
+ * Use case:
+ *     User sets the distance
+ *     User starts the tachymeter when the trip begins
+ *     User stops the tachymeter when the trip ends
+ *     The watch presents the average speed and trip duration in seconds
+ * 
+ * Usage:
+ *     Go to tachymeter face, TC is shown in the Weekday Digits
+ *     A steady d in the Day Digits indicates the distance to be used.
+ *         To edit the distance:
+ *         Long-press the Alarm button, the distance edition page (d will blink)
+ *         Use the Light button to change the editing (blinking) digit, and press Alarm to increase its value
+ *         Once done, long-press the Alarm button to exit the distance edition page
+ *     Press the Alarm button to start the tachymeter.
+ *         A running animation will appear in the Day Digits
+ *     Press the Alarm button to stop the tachymeter
+ *     The average speed and total time information will alternate.
+ *         The average speed will be shown alongside /h in the Day Digits;
+ *         and the total time will be shown alongside t in the Day Digits.
+ *     Long press the Light button to return to the distance d page,
+ *         and restart the tachymeter from there.
+ *     Long-press the light button in the steady distance page to reset
+ *         the distance to 1.00
+ * 
+ * Pending design points
+ * o movement_request_tick_frequency(4) is used to obtain a 4Hz ticking, thus
+ *   having a time resolution of 250 ms. Not sure if using event.subsecond`
+ *   is the proper way to get the fractions of second for the start and
+ *   final times.
+ * o For distance and average speed, the Second Digits (position 8 and 9)
+ *   can be seen as decimals, thus possible to show distances as short as
+ *   0.01 km (or miles) and speeds as low as 0.01 km/h (or mph). However,
+ *   if the same idea is used for the total time (showing hundredths),
+ *   this limits the display to 9999.99 seconds (~2h:45m).
+ */
+
 #include "movement.h"
 
 typedef struct {