include: A ton of little documentation tweaks, fixes, and improvements.

This is just stuff I noticed while working on the wikiheaders updates. A
thorough pass over all the docs would not be terrible, and maybe a simple
script to check for consistency (does everything have a `\since` on it? etc)
might be nice, too.

include/SDL3/SDL_timer.h

@@ -37,9 +37,7 @@
 extern "C" {
 #endif
 
-/**
- * SDL time constants
- */
+/* SDL time constants */
 #define SDL_MS_PER_SECOND   1000
 #define SDL_US_PER_SECOND   1000000
 #define SDL_NS_PER_SECOND   1000000000LL