refers to #945, added comments about legacy and compat var

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | -
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Small description added to the comments on legacy_prefilters and compat_layer variables.