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/nanosec_face.h

@@ -25,6 +25,47 @@
 #ifndef NANOSEC_FACE_H_
 #define NANOSEC_FACE_H_
 
+/*
+ * NANOSEC face
+ *
+ * The goal of nanosec face is dramatic improvement of SensorWatch accuracy.
+ * Minimum goal is <60 seconds of error per year. Full success is if we can
+ * reach <15 seconds per year (<0.47ppm error).
+ *
+ * Best used in conjunction with the FINETUNE face.
+ *
+ * It implements temperature correction using tempco from datasheet (and
+ * allows to adjust these) and allows to introduce offset fix. Therefore
+ * requires temperature sensor board.
+ *
+ * Most users will need to apply profile 3 ("default") or 2 ("conservative
+ * datasheet"), and tune first parameter "static offset" (as it's different
+ * for every crystal sample).
+ *
+ * Frequency correction is dithered over 31 correction intervals (31x10
+ * minutes or ~5 hours), to allow <0.1ppm correction resolution.
+ *  * 1ppm is 0.0864 sec per day.
+ *  * 0.1ppm is 0.00864 sec per day.
+ *
+ * To stay under 1ppm error you would need calibration of your specific
+ * instance of quartz crystal after some "burn-in" (ideally 1 year).
+ *
+ * Should improve TOTP experience.
+ *
+ * Default funing fork tempco: -0.034 ppm/°C², centered around 25°C
+ * We add optional cubic coefficient, which was measured in practice on my sample.
+ *
+ * Cadence (CD) - how many minutes between corrections. Default 10 minutes.
+ * Every minute might be too much. Every hour - slightly less power
+ * consumption but also less precision.
+ *
+ * Can compensate crystal aging (ppm/year) - but you really should be worrying
+ * about it on second/third years of watch calibration.
+ *
+ * For full usage instructions, please refer to the wiki:
+ *  https://www.sensorwatch.net/docs/watchfaces/nanosec/
+ */
+
 #include "movement.h"
 
 #define nanosec_profile_count 5