chore: add docstring to `Simp.Methods.discharge?`

[Paul-Lez]

trafficstars

This PR adds a small docstring to Simp.Methods.discharge? - (this is what I think it does, so any corrections are appreciated:))

This was suggested in the review of this mathlib PR.

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 8ff05f976026b13c6251bfcdb37b4602c0ca6ba2 --onto 2edfe2e9cffd55e3c79291628ae091b942291ec9. You can force Mathlib CI using the force-mathlib-ci label. (2025-04-01 20:28:34)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 8ff05f976026b13c6251bfcdb37b4602c0ca6ba2 --onto da55b2e19b6af219f43f3599f1a72151799eb524. You can force Mathlib CI using the force-mathlib-ci label. (2025-04-04 17:39:24)