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_dialog.h

@@ -46,7 +46,7 @@ extern "C" {
  * \sa SDL_ShowSaveFileDialog
  * \sa SDL_ShowOpenFolderDialog
  */
-typedef struct
+typedef struct SDL_DialogFileFilter
 {
     const char *name;
     const char *pattern;