message: Fix typo and clarify AlphaMapping use. Closes
diff: @@ -444,7 +444,7 @@ written
  // for formats without an alpha channel, define how it should be
  // mapped. Only available for uncompressed simple formats, done
  // in RGBA8 space.
  - DOCUMENT(R"(Controls handling of alpha channel, mostly relevant for file formats that without
  + DOCUMENT(R"(Controls handling of alpha channel, only relevant for file formats that don't have
  alpha.
  It is an :class:`AlphaMapping` that controls what behaviour to use.
  