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_assert.h

@@ -125,6 +125,15 @@ typedef enum
     SDL_ASSERTION_ALWAYS_IGNORE  /**< Ignore the assert from now on. */
 } SDL_AssertState;
 
+/**
+ * Information about an assertion failure.
+ *
+ * This structure is filled in with information about a triggered assertion,
+ * used by the assertion handler, then added to the assertion report.
+ * This is returned as a linked list from SDL_GetAssertionReport().
+ *
+ * \since This struct is available since SDL 3.0.0.
+ */
 typedef struct SDL_AssertData
 {
     SDL_bool always_ignore;