add descriptions to custom fields

[P6g9YHK6]

Is your feature request related to a problem? Please describe.
Currently there is no way of documenting inside TRMM what a field does, where it is used or even how it will be processed.

Describe the solution you'd like
an additional field when creating/editing a custom field to allow for free text with no impact anywhere just for documentation.

Describe alternatives you've considered
using the "default value" field as placeholder for informations about the use of a field

[P6g9YHK6]

Also it would be nice to be able to edit per field the type of icon displayed on the summary tab.