How to give permission for a role for editing help text for fields of custom types?
The Field help helper module seems to be a good fit for this. some more details about it (from its project page, which is also here the image originates from):

The Field help helper module adds a link to entity configurable fields so that users with the appropriate permissions may easily edit the help text and label for a field in context of where the field displays.
Once a user's role is given the Edit field help text permission any user with that permission will be able to edit the help text or label of a configurable field without having access to administer fields.


Extra bonus: comes wth an official release for both D7 and D8 ...