capability: prepare for v0.3.0 release

[kolyshkin]

This is a draft pending merge of

• capability: doc improvements #151
• capability: add LastCap stub for non-Linux #152
• capability: add ListKnown, ListSupported; deprecate List #153

TODO:

• fix the release date.

[tianon]

LGTM (although #151 (comment) applies here too -- alternatively, if they're easier to maintain in append-order instead of prepend-order, maybe we swap 0.1.1 and 0.1.0 and call it a day?)

[kolyshkin]

LGTM (although #151 (comment) applies here too -- alternatively, if they're easier to maintain in append-order instead of prepend-order, maybe we swap 0.1.1 and 0.1.0 and call it a day?)

Thanks, fixed here, too.