Since the word "xtask" is rather difficult to type in my opinion (at least my left hand struggles quite a bit) I wanted to add another alias, `x` as a shorthand (similar to what `build`, `run`, etc. have by default). Thereby I discovered that I needn't replicate the whole alias, because aliases are recursive. I consulted the docs and couldn't find a mention of this, hence I'm adding it as part of this PR so other users can discover it.
### How should we test and review this PR?
I don't think this requires a separate test, it's a minor change to the documentation only.