docs: add lock file section to the resolver docs

[Fishrock123]

Adds some documentation about how lockfiles work to the resolver section, particularly regarding when lock files exist in library dependencies.

Effectively re-adds an updated section of docs that was lost here: https://github.com/rust-lang/cargo/pull/12382/files#diff-197a732275c32bdbdb079bdd92ac8a4ba585ee556ea978e9e661804eb76ce9eeL117-L121

Related to this Zulip thread https://rust-lang.zulipchat.com/#narrow/channel/246057-t-cargo/topic/cargo.20lock.20in.20dependency/with/519679182

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[weihanglo]

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[epage]

For myself, I would try to fit lockfiles into a new section under Constraints and Heuristics. Unsure if I'm losing too much nuance but we can describe lockfiles as being relevant to pick_next_version, preferring the locked version over all else.

[epage]

For myself, I would try to fit lockfiles into a new section under Constraints and Heuristics. Unsure if I'm losing too much nuance but we can describe lockfiles as being relevant to pick_next_version, preferring the locked version over all else.

Did this in #15958.