Cleanup: move public doc-strings into headers for 'blenlib'

- Added space below non doc-string comments to make it clear
  these aren't comments for the symbols directly below them.
- Use doxy sections for some headers.
- Minor improvements to doc-strings.

Ref T92709

source/blender/blenlib/BLI_math_time.h

@@ -27,7 +27,10 @@
 extern "C" {
 #endif
 
-/************************ Time constants definitions***************************/
+/* -------------------------------------------------------------------- */
+/** \name Time Constants Definitions
+ * \{ */
+
 #define SECONDS_IN_MILLISECONDS 0.001
 #define SECONDS_IN_MINUTE 60.0
 #define MINUTES_IN_HOUR 60.0
@@ -37,6 +40,15 @@ extern "C" {
 #define SECONDS_IN_DAY (MINUTES_IN_DAY * SECONDS_IN_MINUTE)
 #define SECONDS_IN_HOUR (MINUTES_IN_HOUR * SECONDS_IN_MINUTE)
 
+/** \} */
+
+/** Explode given time value expressed in seconds, into a set of days, hours, minutes, seconds
+ * and/or milliseconds (depending on which return parameters are not NULL).
+ *
+ * \note The smallest given return parameter will get the potential fractional remaining time
+ * value. E.g. if you give `seconds=90.0` and do not pass `r_seconds` and `r_milliseconds`,
+ * `r_minutes` will be set to `1.5`.
+ */
 void BLI_math_time_seconds_decompose(double seconds,
                                      double *r_days,
                                      double *r_hours,
@@ -44,7 +56,15 @@ void BLI_math_time_seconds_decompose(double seconds,
                                      double *r_seconds,
                                      double *r_milliseconds);
 
-/**************************** Inline Definitions ******************************/
+/** \} */
+
+/* -------------------------------------------------------------------- */
+/** \name Inline Definitions
+ * \{ */
+
+/* None. */
+
+/** \} */
 
 #ifdef __cplusplus
 }