Clarify altersDynamicFields should be set for custom fields no matter what

[BrunoBernardino]

In https://zapier.github.io/zapier-platform-cli/#dynamic-dropdowns we say:

Dynamic dropdowns are one of the few fields that automatically invalidate Zapier's field cache, so it is not necessary to set altersDynamicFields to true for these fields.

That's true only for non-custom fields, so we should make that clearer.

Basically, if your static field is a dynamic dropdown but doesn't have altersDynamicFields, and it alters custom fields, it needs to be set regardless.