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_error.h

@@ -122,7 +122,7 @@ extern DECLSPEC void SDLCALL SDL_ClearError(void);
 #define SDL_OutOfMemory()   SDL_Error(SDL_ENOMEM)
 #define SDL_Unsupported()   SDL_Error(SDL_UNSUPPORTED)
 #define SDL_InvalidParamError(param)    SDL_SetError("Parameter '%s' is invalid", (param))
-typedef enum
+typedef enum SDL_errorcode
 {
     SDL_ENOMEM,
     SDL_EFREAD,
@@ -133,7 +133,7 @@ typedef enum
 } SDL_errorcode;
 
 /**
- * SDL_Error()
+ * Set an SDL error from a list of error codes.
  *
  * \param code Error code
  * \returns unconditionally -1.