Sync SDL3 wiki -> header

[ci skip]

(cherry picked from commit 2e18cf8d096657be40ab301a523e2f10a2a10508)

include/SDL3/SDL_stdinc.h

@@ -1816,6 +1816,11 @@ extern SDL_DECLSPEC const char * SDLCALL SDL_getenv(const char *name);
  * This function bypasses SDL's cached copy of the environment and is not
  * thread-safe.
  *
+ * On some platforms, this may make case-insensitive matches, while other
+ * platforms are case-sensitive. It is best to be precise with strings used
+ * for queries through this interface. SDL_getenv is always case-sensitive,
+ * however.
+ *
  * \param name the name of the variable to get.
  * \returns a pointer to the value of the variable or NULL if it can't be
  *          found.