[docs] minor fix & update

USAGE.md

@@ -321,7 +321,7 @@ unsigned notcurses_palette_size(const struct notcurses* nc);
 bool notcurses_canfade(const struct notcurses* nc);
 
 // Can we directly specify RGB values per cell, or only use palettes?
-bool notcurses_can_truecolor(const struct notcurses* nc);
+bool notcurses_cantruecolor(const struct notcurses* nc);
 
 // Can we load images? This requires being built against FFmpeg/OIIO.
 bool notcurses_canopen_images(const struct notcurses* nc);

include/notcurses/notcurses.h

@@ -333,6 +333,8 @@ ncchannels_set_fchannel(uint64_t* channels, uint32_t channel){
   return *channels = (*channels & 0xfffffffflu) | ((uint64_t)channel << 32u);
 }
 
+// Creates a new channel pair using 'fchan' as the foreground channel
+// and 'bchan' as the background channel.
 static inline uint64_t
 ncchannels_combine(uint32_t fchan, uint32_t bchan){
   uint64_t channels = 0;