Add documentation to disable ktlint No Unit returns errors

[Tylos]

References

Related to #1386 and #1385 effort to document how to disable different linting errors that do not make sense for Arrow

Description

Apart from the IntellIJ IDEA Inspection, Ktlint also complaints about Unit return types regardless of where they're used. That's also something we could suggest users how to disable on their ktlint configurations, since from an FP perspective it's always important to declare return types regardless of what they are, so it's explicit that the function has side effects inside

However, at the moment Ktlint does not support disabling rules. There is current work in progress to include an easy way to disable rules at a project level which would solve the problem in this issues - pinterest/ktlint#503