Docs: Add missing doc for HEADER_GUARD option

Amends f3125dd7a3.

Task-number: QTBUG-125406
Change-Id: Id0000d8a0d1481334cd7c24df504c269dd0d046a
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>

src/grpc/doc/src/qt-add-grpc.qdoc

@@ -24,6 +24,7 @@ Usually \c qtgrpcgen is invoked through the \c qt_add_grpc CMake macro.
         [EXPORT_MACRO <infix>]
         [OUTPUT_HEADERS <var>]
         [OUTPUT_TARGETS <var>]
+        [HEADER_GUARD <string>]
     )
 \endcode
 
@@ -84,6 +85,7 @@ target to the protobuf one:
     \include qtprotoccommon.qdocinc export_macro-cmake-li
     \include qtprotoccommon.qdocinc output_headers-li
     \include qtprotoccommon.qdocinc output_targets-li
+    \include qtprotoccommon.qdocinc header_guard-li
 \endlist
 
 \sa{The qtgrpcgen Tool}

src/grpc/doc/src/qtgrpcgen.qdoc

@@ -64,6 +64,7 @@ options are supported:
     \include qtprotoccommon.qdocinc copy_comments-li
     \include qtprotoccommon.qdocinc generate_package_subfolders-li
     \include qtprotoccommon.qdocinc export_macro-cli-li
+    \include qtprotoccommon.qdocinc header_guard-li
     \include qtgrpcgen.qdocinc qml-li
 \endlist
 */

src/protobuf/doc/src/qt-add-protobuf.qdoc

@@ -26,6 +26,7 @@ invoked through CMake using the \c qt_add_protobuf command.
         [EXPORT_MACRO <infix>]
         [OUTPUT_HEADERS <var>]
         [OUTPUT_TARGETS <var>]
+        [HEADER_GUARD <string>]
     )
 \endcode
 
@@ -108,6 +109,7 @@ link.
     \include qtprotoccommon.qdocinc export_macro-cmake-li
     \include qtprotoccommon.qdocinc output_headers-li
     \include qtprotoccommon.qdocinc output_targets-li
+    \include qtprotoccommon.qdocinc header_guard-li
 \endlist
 
 \sa{The qtprotobufgen Tool}

src/protobuf/doc/src/qtprotobufgen.qdoc

@@ -67,5 +67,6 @@ options are supported:
     \include qtprotoccommon.qdocinc copy_comments-li
     \include qtprotoccommon.qdocinc generate_package_subfolders-li
     \include qtprotoccommon.qdocinc export_macro-cli-li
+    \include qtprotoccommon.qdocinc header_guard-li
 \endlist
 */

src/tools/doc/qtprotoccommon.qdocinc

@@ -71,3 +71,10 @@
     specifies a variable that will store the list of generated targets. This
     list can be useful for defining custom project install rules.
 //! [output_targets-li]
+
+//! [header_guard-li]
+\li \c {HEADER_GUARD}
+    specifies the mechanism used for guarding generated header files from
+    multiple inclusion. Possible values are \c {<pragma | filename>}. Defaults
+    to \c {filename}
+//! [header_guard-li]