Update doxygen annotations in libosmocore

This adds and improves doxygen API descriptions all over libosmocore,
reducing the 'white spots' that don't have any documentation.
diff --git a/src/timer.c b/src/timer.c
index 21b7015..0358b89 100644
--- a/src/timer.c
+++ b/src/timer.c
@@ -249,6 +249,8 @@
 	return work;
 }
 
+/*! \brief Check how many timers we have in the system
+ *  \returns number of \ref osmo_timer_list registered */
 int osmo_timers_check(void)
 {
 	struct rb_node *node;