docs: clarify the coverage-exclude and token-seed comments

The Makefile's `COVERAGE_EXCLUDE' comment said why `test-all-comp-errors.el' is excluded but not why `test-lorem-optimum-benchmark.el' is. It now notes that undercover's instrumentation slows execution enough to fail the benchmark's wall-clock assertions. And `cj/markov-generate' now has a comment explaining why `tokens' is seeded reversed (`(list w2 w1)'): the list is built with `push' and `nreverse'd at the end, so without the note the reversed seed reads like a bug at a glance.

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