Improve configuration form texts

Problem/Motivation

On first usage, i do not understand the config checkboxes easily:
- 1 Automatically accept invitation (no description) - what does this mean?
- 2 Unblock registered users coming from an invitation (no description) - I have an idea, but not quite.
- 3 Accept invitations immediately / When accepting an invitation, the group membership entity form will be rendered. Enabling this option will bypass this step and the membership will be generated immediately. This is especially useful if your membership entity provides no configuration at all and the invitation accept route renders an empty form with a single submit button. - I can understand this from the description, but the label is misleading.

Proposed resolution

Improve tests / descriptions
- 1 Add description: When a user registers with an email matching an invitation, spare them to go the to their 'My Invitations' list, and accept the invitation.
- 2 Add description: When a user registers with an email matching an invitation, unblock it with no additional user administration action.
- 3: Change label to: Skip invitation creation form OR Skip invitation form