Doc: Qt Core: Increase warning limit to 2

Technically the Qt Core documentation builds without warnings, but
testing in CI two warnings are observed:

  warning: Already generated qtcore-attribution-extra-cmake-modules.html
           for this project
  warning: Already generated qtcore-attribution-kwin.html
           for this project

These pages come from a qtattributionsscanner-generated source file,
codeattributions.qdoc.

The warnings do not affect the generated output. Increase the
documentation warning limit temporarily to let integrations pass.

Pick-to: 6.8
Task-number: QTBUG-126546
Change-Id: Ic8641c69b2818c72af72cdc23161314caf376c27
Reviewed-by: Paul Wicking <paul.wicking@qt.io>

src/corelib/doc/qtcore.qdocconf

@@ -66,5 +66,5 @@ manifestmeta.highlighted.names = \
     "QtCore/Serialization Converter" \
     "QtCore/QXmlStream Bookmarks Example"
 
-# Temporarily allow warning: No such parameter 'other' in QFlags::operator=()
-warninglimit = 1
+# QTBUG-126546
+warninglimit = 2