doc: Improve QTextTableFormat::setBorderCollapse and related docs

We changed the default in 6.8. It was documented in the changelog but
not in docs. Amends 09ea47f811

Fixes: QTBUG-132173
Change-Id: I1b6bf69931ba12025cf95e80c12a5073a94af598
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
(cherry picked from commit 622133ace2533617f294b8d875b5eb749a78571b)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit a4dc60e9fcf5e5131d620240bb0abdca9911e82c)

src/gui/text/qtextformat.cpp

@@ -3291,7 +3291,7 @@ QTextTableFormat::QTextTableFormat(const QTextFormat &fmt)
     \fn void QTextTableFormat::setBorderCollapse(bool borderCollapse)
     \since 5.14
 
-    Enabling \a borderCollapse will have the following implications:
+    By default, \l borderCollapse() is \c true, which has the following implications:
     \list
     \li The borders and grid of the table will be rendered following the
         CSS table \c border-collapse: \c collapse rules
@@ -3308,10 +3308,12 @@ QTextTableFormat::QTextTableFormat(const QTextFormat &fmt)
         \endlist
     \endlist
 
-    With borderCollapse disabled, cell borders can still be styled
+    With \a borderCollapse set to \c false, cell borders can still be styled
     using QTextTableCellFormat but styling will be applied only within
     the cell's frame, which is probably not very useful in practice.
 
+    \note In Qt versions prior to 6.8, the default value was \c false.
+
     \sa setBorder(), setBorderBrush(), setBorderStyle()
     \sa QTextTableCellFormat
 */
@@ -3320,7 +3322,7 @@ QTextTableFormat::QTextTableFormat(const QTextFormat &fmt)
     \fn bool QTextTableFormat::borderCollapse() const
     \since 5.14
 
-    Returns true if borderCollapse is enabled.
+    Returns \c true if table borders are to be collapsed. The default is \c true.
 
     \sa setBorderCollapse()
 */