Only show description about allowed targets

Problem/Motivation

Thanks for the awesome module. Definitely covered the need we had on our project.
But when allowing to specify what targets are available for selection, it is not instructive to show the description about all targets. It would be better to only show the description about the targets that are allowed.

Steps to reproduce

Install module
Enable the field widget for a link field
Only allow a couple of targets, not all
See the description being for all targets.

Proposed resolution

Split up the description so it can be checked for what targets the description can be shown.

Remaining tasks

Create MR
Review MR
Merge Mr

User interface changes

The description will change to only show the allowed targets.

API changes

None

Data model changes

None