include: Fill in more category documentation.

Reference Issue #11874.

include/SDL3/SDL_stdinc.h

@@ -22,11 +22,25 @@
 /**
  * # CategoryStdinc
  *
- * This is a general header that includes C language support. It implements a
- * subset of the C runtime APIs, but with an `SDL_` prefix. For most common
- * use cases, these should behave the same way as their C runtime equivalents,
- * but they may differ in how or whether they handle certain edge cases. When
- * in doubt, consult the documentation for details.
+ * SDL provides its own implementation of some of the most important C runtime
+ * functions.
+ *
+ * Using these functions allows an app to have access to common C
+ * functionality without depending on a specific C runtime (or a C runtime at
+ * all). More importantly, the SDL implementations work identically across
+ * platforms, so apps can avoid surprises like snprintf() behaving differently
+ * between Windows and Linux builds, or itoa() only existing on some
+ * platforms.
+ *
+ * For many of the most common functions, like SDL_memcpy, SDL might just
+ * call through to the usual C runtime behind the scenes, if it makes sense to
+ * do so (if it's faster and always available/reliable on a given platform),
+ * reducing library size and offering the most optimized option.
+ *
+ * SDL also offers other C-runtime-adjacent functionality in this header that
+ * either isn't, strictly speaking, part of any C runtime standards, like
+ * SDL_crc32() and SDL_reinterpret_cast, etc. It also offers a few better
+ * options, like SDL_strlcpy(), which functions as a safer form of strcpy().
  */
 
 #ifndef SDL_stdinc_h_