Implement base paths (RFC 3529) 2/n: unstable docs

[dpaoliello]

RFC: rust-lang/rfcs#3529
Tracking Issue: #14355

This PR documents the feature in the unstable book.

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[epage]

imo we shouldn't merge functionality without the docs (except in some very specific circumstances). This fully belongs with the prior PR. I think what got confusing was my comment about the unstable docs. I was more suggesting something like b8a6846 or 4d09a6a (though in its own PR rather than a commit in a PR. Then as each PR adds functionality, the docs get updated to reflect that.

[dpaoliello]

Merging with implementation PRs