feat(algebra/ordered_monoid_lemmas): add one `strict_mono` lemma and a few doc-strings (#8465)
feat(algebra/ordered_monoid_lemmas): add one `strict_mono` lemma and a few doc-strings (#8465)

The product of strictly monotone functions is strictly monotone (and some doc-strings).

[Zulip discussion](https://leanprover.zulipchat.com/#narrow/stream/217875-Is-there.20code.20for.20X.3F/topic/monotonicity.20for.20sums.20and.20products.20of.20monotone.20functions)





leanprover-community/mathlib,leanprover-community/mathlib