* Correct fmt -check
With `-check=false` the exit status is always zero.
With `-check=true` the exit status is zero when all files are properly formatted and non-zero otherwise.
* update fmt documentation to use short form for -diff and -check
We have released the v0.12-oriented content to the website early in order
to support the beta process, but in some places we neglected to explicitly
mark features or content as being v0.12-only.
Here we add explicit markers to the main cases we've seen where readers
have reported confusion, along with some other tweaks in similar vein.
Stefan Schmidt