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.

src/hidapi/SDL_hidapi.c

@@ -998,7 +998,7 @@ static const struct hidapi_backend LIBUSB_Backend = {
 };
 #endif /* HAVE_LIBUSB */
 
-struct SDL_hid_device_
+struct SDL_hid_device
 {
     const void *magic;
     void *device;