docs: document about which algorithms support `length` option for stdCrypto.digest

[kt3k]

related: https://github.com/denoland/deno_std/pull/1386

[iuioiua]

@kt3k, I believe this can be closed.

[lino-levan]

@iuioiua The bug has been fixed but the docs have not been changed.

@kt3k How should this be documented? Is this just a JSDoc comment?

[kt3k]

Let's document in JSDoc comment. I also think it's better to document when length is meaningful. Do all digest algorithms respect length property? (I guess no)