stb_image: Better docs for stbi_info.

Fixes #1026.

stb_image.h

@@ -1,4 +1,4 @@
-/* stb_image - v2.26 - public domain image loader - http://nothings.org/stb
+/* stb_image - v2.27 - public domain image loader - http://nothings.org/stb
                                   no warranty implied; use at your own risk
 
    Do this:
@@ -48,6 +48,7 @@ LICENSE
 
 RECENT REVISION HISTORY:
 
+      2.27  (          ) document stbi_info better
       2.26  (2020-07-13) many minor fixes
       2.25  (2020-02-02) fix warnings
       2.24  (2020-02-02) fix warnings; thread-local failure_reason and flip_vertically
@@ -176,6 +177,14 @@ RECENT REVISION HISTORY:
 //
 // Paletted PNG, BMP, GIF, and PIC images are automatically depalettized.
 //
+// To query the width, height and component count of an image without having to
+// decode the full file, you can use the stbi_info family of functions:
+//
+//   int x,y,n,ok;
+//   ok = stbi_info(filename, &x, &y, &n);
+//   // returns ok=1 and sets x, y, n if image is a supported format,
+//   // 0 otherwise.
+//
 // ===========================================================================
 //
 // UNICODE: