Issue #3154240: Clarify labels for "never logged in" and "not logged in" fieldsets (4.x backport)

Backport of MR !57 to the 4.x branch.

Clarifies the fieldset labels in the Purge Users settings form and adds a #description on each so it is explicit that:

• "never logged in" is counted from the account creation date;
• "not logged in" is counted from the last login date.

Refs #3154240.