qmake: document {,QMAKE_}LIBS_PRIVATE

Change-Id: I6637d5f57d372f63b3cfaeb28c7e48c940887db6
Reviewed-by: Liang Qi <liang.qi@qt.io>
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

qmake/doc/src/qmake-manual.qdoc

@@ -1320,6 +1320,19 @@
 
     \snippet code/doc_src_qmake-manual.pro 39
 
+    \target LIBS_PRIVATE
+    \section1 LIBS_PRIVATE
+
+    Specifies a list of libraries to be linked privately into the project.
+    The behavior of this variable is identical to \l LIBS, except that
+    shared library projects built for Unix do not expose these dependencies
+    in their link interface.
+
+    The effect of this is that if project C depends on library B which
+    depends on library A privately, but C also wants to use symbols from A
+    directly, it needs to link to A explicitly. Put differently, libraries
+    linked privately are not exposed transitively at build time.
+
     \target LITERAL_HASH
     \section1 LITERAL_HASH
 
@@ -2104,6 +2117,16 @@
 
     To specify libraries in a project file, use \l LIBS instead.
 
+    \section1 QMAKE_LIBS_PRIVATE
+
+    Specifies additional private libraries each project needs to
+    link against.
+    The value of this variable is typically handled by qmake or
+    \l{#QMAKESPEC}{qmake.conf} and rarely needs to be modified.
+
+    To specify private libraries in a library project file,
+    use \l LIBS_PRIVATE instead.
+
     \section1 QMAKE_LIBS_EGL
 
     Specifies all EGL libraries when building Qt with OpenGL/ES