doc: Make docs for ElideNone a bit more precise

ElideNone was originally introduced as a way of modifying behavior
of specific widgets, such as QTabBar. The text would previously
always be a elided and an ellipsis character would be added somewhere
to indicate this to the user. Given that context, it makes sense
that the documentation says the effect of ElideNone is that ellipsis
does not appear in the text, since the text would still be elided
if it were too long, given that the painter would draw it clipped to
the widget.

When QFontMetrics::elidedText() was introduced later, this became more
confusing. In the context of this function, you would expect the text
to be returned truncated to the width but without the ellipsis character.
Instead it just returns the full string.

Since this has been the effect of ElideNone since the start and multiple
places use it as a way of disabling the truncation of the string, we
simply update the documentation to make this more clear.

Pick-to: 5.15 6.2 6.5 6.6
Fixes: QTBUG-40315
Change-Id: If037430da6260020b1d99be7bd43e622f8afa844
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/corelib/global/qnamespace.qdoc

@@ -2874,7 +2874,10 @@
     \value ElideLeft    The ellipsis should appear at the beginning of the text.
     \value ElideRight   The ellipsis should appear at the end of the text.
     \value ElideMiddle  The ellipsis should appear in the middle of the text.
-    \value ElideNone    Ellipsis should NOT appear in the text.
+    \value ElideNone    Ellipsis should NOT appear in the text. When passed to functions such as
+                        QFontMetrics::elidedText(), this will cause the full string to return unless
+                        the text contains multi-length variants. Elision in this case must be done
+                        by clipping to the component width.
 
     Qt::ElideMiddle is normally the most appropriate choice for URLs (e.g.,
     "\l{http://bugreports.qt.io/browse/QTWEBSITE-13}{http://bugreports.qt.../QTWEBSITE-13/}"),