Document Tab.for_keg and use Keg#tab where possible.

[reitermarkus]

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[carlocab]

Keg#tab just calls Tab.for_keg so I'm not sure why you would prefer Keg#tab over Tab.for_keg.

[reitermarkus]

Because it's shorter.

[reitermarkus]

Also, to better keep track of where Tab.for_keg is still called with a Pathname, so we can eventually change it to only take Kegs.

[MikeMcQuaid]

Looks good, thanks again @reitermarkus!