add comments for ncpixel functions

include/notcurses/notcurses.h

@@ -2361,6 +2361,8 @@ API int ncblit_bgrx(const void* data, int linesize,
 // The ncpixel API facilitates direct management of the pixels within an
 // ncvisual (ncvisuals keep a backing store of 32-bit RGBA pixels, and render
 // them down to terminal graphics in ncvisual_render()).
+
+// Extract the 8-bit alpha component from a pixel
 static inline uint32_t
 ncpixel(int r, int g, int b){
   if(r < 0) r = 0;
@@ -2372,26 +2374,31 @@ ncpixel(int r, int g, int b){
   return 0xff000000ul | r | (b << 8u) | (g << 16u);
 }
 
+// Extract the 8-bit alpha component from a pixel
 static inline unsigned
 ncpixel_a(uint32_t pixel){
   return (pixel & 0xff0000fful) >> 24u;
 }
 
+// Extract the 8 bit red component from a pixel
 static inline unsigned
 ncpixel_r(uint32_t pixel){
   return (pixel & 0x000000fful);
 }
 
+// Extract the 8 bit green component from a pixel
 static inline int
 ncpixel_g(uint32_t pixel){
   return (pixel & 0x00ff0000ul) >> 16u;
 }
 
+// Extract the 8 bit blue component from a pixel
 static inline int
 ncpixel_b(uint32_t pixel){
   return (pixel & 0x0000ff00ul) >> 8u;
 }
 
+// Set the 8-bit alpha component of a pixel
 static inline int
 ncpixel_set_a(uint32_t* pixel, int a){
   if(a > 255 || a < 0){
@@ -2401,6 +2408,7 @@ ncpixel_set_a(uint32_t* pixel, int a){
   return 0;
 }
 
+// Set the 8-bit red component of a pixel
 static inline int
 ncpixel_set_r(uint32_t* pixel, int r){
   if(r > 255 || r < 0){
@@ -2410,6 +2418,7 @@ ncpixel_set_r(uint32_t* pixel, int r){
   return 0;
 }
 
+// Set the 8-bit green component of a pixel
 static inline int
 ncpixel_set_g(uint32_t* pixel, int g){
   if(g > 255 || g < 0){
@@ -2419,6 +2428,7 @@ ncpixel_set_g(uint32_t* pixel, int g){
   return 0;
 }
 
+// Set the 8-bit blue component of a pixel
 static inline int
 ncpixel_set_b(uint32_t* pixel, int b){
   if(b > 255 || b < 0){