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/settings/finetune_face.h

@@ -25,6 +25,34 @@
 #ifndef FINETUNE_FACE_H_
 #define FINETUNE_FACE_H_
 
+/*
+ * FINETUNE face
+ *
+ * FineTune face allows to align watch with sub-second precision in 25/250ms
+ * accuracy. Counts time since previous finetune, and allows to calculate &
+ * apply ppm correction for nanosec.
+ *
+ * Best used in conjunction with the NANOSEC face.
+ *
+ * Main screen - adjust delay (light/alarm)
+ * Long MODE press - show hours since previous finetune
+ * Long MODE press - show calculated ppm correction.
+ *  You can apply it with long LIGHT, or just reset finetune timer with long ALARM.
+ *
+ * Finetune will apply crystal aging correction on every finetune save
+ * (as aging is calculated since "last finetune" timestamp); but you should
+ * worry about aging only on second/third years of watch calibration (if you
+ * are really looking at less than 10 seconds per year of error).
+ *
+ * Warning, do not use at the first second of a month, as you might stay at
+ * the same month and it will surprise you. Just wait 1 second...We are not
+ * fully replicating RTC timer behavior when RTC is off.
+ * Simulating months and years is... too much complexity.
+ *
+ * For full usage instructions, please refer to the wiki:
+ *  https://www.sensorwatch.net/docs/watchfaces/nanosec/
+ */
+
 #include "movement.h"
 
 typedef struct {