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/stopwatch_face.h

@@ -26,6 +26,17 @@
 #ifndef STOPWATCH_FACE_H_
 #define STOPWATCH_FACE_H_
 
+/*
+ * STOPWATCH FACE
+ *
+ * The Stopwatch face provides basic stopwatch functionality: you can start
+ * and stop the stopwatch with the alarm button. Pressing the light button
+ * when the timer is stopped resets it.
+ *
+ * This face does not count sub-seconds.
+ * See also: "stock_stopwatch_face.h"
+ */
+
 #include "movement.h"
 
 typedef struct {