[doc] QFormDataPartBuilder: mark private build() as \internal

It makes no sense to document (non-virtual) private functions.

Change-Id: Ifeed52ea5e9a5b8e358277140419c8d68204a45b
Reviewed-by: Juha Vuolle <juha.vuolle@qt.io>
(cherry picked from commit 19b7a08fc24c5947fb1ea921e3d24f4efce4d1fb)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/network/access/qformdatabuilder.cpp

@@ -186,6 +186,8 @@ QFormDataPartBuilder &QFormDataPartBuilder::setHeaders(const QHttpHeaders &heade
 }
 
 /*!
+    \internal
+
     Generates a QHttpPart and sets the content disposition header as form-data.
 
     When this function called, it uses the MIME database to deduce the type the