Environment linters should remind users how to disable them

[bryangingechen]

Similarly to how syntax linters in Lean core do it: https://github.com/leanprover/lean4/blob/206eb73cd9de4b58abbb2eae20a41fd9b7af187b/src/Lean/Linter/Basic.lean#L75

The relevant code seems to be at:

batteries/Batteries/Tactic/Lint/Frontend.lean

Line 194 in 6f7d05f

s := m!"-- Found {failed} error{if failed == 1 then "" else "s"

cf. #FLT > unused argument linter issue @ 💬

(It might also be good for the documentation of environment linters to mention the term "environment linter" and disucss the distinction with "syntax linters" as well).