Doc: Add doc for the \dontdocument command

Task-number: QTBUG-78936
Change-Id: Ia8635aa1b2d2ce97e5c1390bcdd7a5c920d87e83
Reviewed-by: Martin Smith <martin.smith@qt.io>

src/qdoc/doc/qdoc-manual-contextcmds.qdoc

@@ -237,6 +237,24 @@
     See how QDoc renders this property on the reference page for the
     \l {State::changes}{State} type.
 
+    \target dontdocument-command
+    \section1 \\dontdocument
+
+    The \\dontdocument command is only used in a dontdocument.qdoc file
+    for a particular module.  This file specifies publically declared
+    classes or structs that are not meant to be documented. Then QDoc will
+    not print warnings about missing \class comments for these classes
+    and structs.
+
+    Below you will find the \\dontdocument command in the
+    dontdocument.qdoc for widgets:
+
+    \code
+    /*!
+       \dontdocument (QTypeInfo QMetaTypeId)
+    */
+    \endcode
+
     \target obsolete-command
     \section1 \\obsolete