Rollup merge of #146373 - calvinhirsch:trait-solver-cycle-head-docs, r=lcnr

fix comments about trait solver cycle heads

update some comments that use "cycle root" rather than "cycle head"
also fixed a random other nearby typo in `StackEntry` docs

zulip thread: https://rust-lang.zulipchat.com/#narrow/channel/364551-t-types.2Ftrait-system-refactor/topic/quick.20question.20about.20SearchGraph.3A.3Aevaluate_goal_in_task/with/538004295

?r lcnr

Author: matthiaskrgr