QDoc: Fix shared-comment since descriptions

A shared comment with one member is introduced as "These ... were"
in HTML and DocBook. DocBook also forms plurals by appending s,
which produces incorrect labels for properties and classes. Select
the subject and node description by the number of members instead.

Pick-to: 6.12
Task-number: QTBUG-151329
Change-Id: I2f4a1e23075eb9a0a7dd9e2866c51fd5f34cfd0c
Reviewed-by: David Boddie <david.boddie@qt.io>