Like A.T.S.F said the verbage you are looking for in your lease should be under maintenance and repairs.
Something along the line that if any required repair is caused by the negligence of the tenant and/or tenant's guest, the tenant will be fully responsible for the cost of the repair and or replacement that may be needed.
Also your lease should state that the property is to be returned to the landlord in the same condition that it was given.
A shower door shattering is negligence. Glass does not shatter unless it was slammed, hit or somehow used in a manner that it was not intended to be used.
I would remind them that there was no mention of cracking on the shower door so they are responsible for replacing it. Had they reported an issue with the door previously it would be a different story. Now if they have been there awhile and are great tenants you may want to consider replacing it or if they raise a fuss put in a shower rod for them.