[Maintenance] [th01] .GRP: Describe the format

Part of P0229, funded by Ember2528.

th01/formats/grp.h

@@ -1,3 +1,21 @@
+/// .PI variant
+/// -----------
+/// .PI is a common lossless PC-98 image format that features efficient
+/// compression for the often dithered 16-color pixel art typically associated
+/// with the platform. This .GRP format only differs from regular .PI files in
+/// two minor aspects:
+///
+/// • The two magic header bytes are "ZN" instead of "Pi". (Lol)
+/// • The palette uses PC-98-native 4-bit color component values from 0x0 to
+///   0xF, leaving the top 4 bits empty. .PI loaders that expect 8-bit color
+///   components will therefore interpret .GRP images as being very dark.
+///
+/// These functions are just wrappers around the PiLoad library, which
+/// implements the actual format by directly decoding an image to VRAM.
+/// All .GRP images are assumed to be 640×400.
+///
+/// More info on the original format: https://mooncore.eu/bunny/txt/pi-pic.htm
+
 // Always updated by any of the .GRP loading or blitting functions.
 extern Palette4 grp_palette;
 
@@ -37,3 +55,4 @@ int grp_put_colorkey(const char *fn);
 	void pascal grp_palette_white_out(unsigned int frames);
 	void pascal grp_palette_white_in(unsigned int frames);
 #endif
+/// -----------