doc: fix ensuareAlpha alpha argument

[gutenye]

1 is full-transparent

[lovell]

I'm unsure I understand this change as the current documentation is correct. Adding an alpha channel using a value of 0 produces a fully-transparent image.

The alpha channel relates to the ability to see whatever is behind the image. Perhaps you are thinking of a mask?

[lovell]

Thanks for taking the time to open a PR - I'll close as the documentation is already correct in this regard.

[gutenye]

@lovell you're right, the output file must be png.

If I use jpg as output, like below, it outputs a black image, what caused it? This gets me confused.

await sharp('rgb.jpg')
  .ensureAlpha(0)
  .toFile('output.jpg')

[lovell]

JPEG images do not support an alpha channel.