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_locale.h

@@ -39,7 +39,18 @@ extern "C" {
 /* *INDENT-ON* */
 #endif
 
-
+/**
+ * A struct to provide locale data.
+ *
+ * Locale data is split into a spoken language, like English, and an
+ * optional country, like Canada. The language will be in ISO-639 format
+ * (so English would be "en"), and the country, if not NULL, will be an
+ * ISO-3166 country code (so Canada would be "CA").
+ *
+ * \since This function is available since SDL 3.0.0.
+ *
+ * \sa SDL_GetPreferredLocales
+ */
 typedef struct SDL_Locale
 {
     const char *language;  /**< A language name, like "en" for English. */