QDoc: Pluralize deprecation notes for shared docs

When one documentation comment covers several members and marks them
`\deprecated`, QDoc says "This function is deprecated" and advises
against using "it". The singular wording reads oddly under a list of
several signatures. This leaves readers unsure whether the notice
applies to every member in the list. The note for members scheduled
for deprecation in a later version has the same problem.

This change makes QDoc use the plural form for such comments in HTML and
DocBook output, for example "These functions are deprecated" and "using
them". This matches how QDoc words the since-version note for the
same comments. A property group keeps the singular form. It documents
one property with several parts, not several independent members.

Pick-to: 6.12
Fixes: QTBUG-151319
Change-Id: I7ad7844bc8a178d0d21622493e1838539145cc4b
Reviewed-by: Topi Reinio <topi.reinio@qt.io>