Document `BAZELISK_NOJDK` and `BAZELISK_FORMAT_URL` for .bazeliskrc

[albertocavalcante]

Besides BAZELISK_NOJDK, I noticed BAZELISK_FORMAT_URL was not explicitly mentioned as supported to be set on .bazeliskrc so I've also included it.

Fixes https://github.com/bazelbuild/bazelisk/issues/584

(As this is my first contribution please let me know if you need to perform any CCLA checks.)

[albertocavalcante]

Hey @meteorcloudy, just wondering if I should tag anyone to review. Thanks.

[meteorcloudy]

@fweikert Can you take a look?

[fweikert]

Thanks!