[Qt module] Privates headers: added documentation

Signed-off-by: Alexis Jeandet <alexis.jeandet@member.fsf.org>

docs/markdown/Qt5-module.md

@@ -20,6 +20,7 @@ This method generates the necessary targets to build translation files with lrel
  - `install_dir` directory to install to (optional).
  - `build_by_default` when set to true, to have this target be built by default, that is, when invoking plain ninja; the default value is false (optional).
 
+## Example
 A simple example would look like this:
 
 ```meson
@@ -38,3 +39,9 @@ executable('myprog', 'main.cpp', 'myclass.cpp', moc_files,
 
 The 'modules' argument is used to include Qt modules in the project.
 See the Qt documentation for the [list of modules](http://doc.qt.io/qt-5/qtmodules.html).
+
+## private headers (since v0.47.0)
+
+Some projects needs Qt's private headers to build, that's why a **private_headers** keyword argument has been added to [dependency](Reference-manual.md#dependency) method.
+Setting this optionnal argument will add private include path of the given module to the compiler flags.
+Note that this option is only compatible with qmake dependency method, using auto or pkg-config will fallback to qmake.

docs/markdown/Reference-manual.md

@@ -341,6 +341,7 @@ otherwise. This function supports the following keyword arguments:
   `>1.0.0`, `<=2.3.5` or `3.1.4` for exact matching. (*Added 0.37.0*)
   You can also specify multiple restrictions by passing a list to this
   keyword argument, such as: `['>=3.14.0', '<=4.1.0']`.
+- `private_headers`, only available with Qt modules see [documentation](Qt5-module.md#private_headers).
 
 If dependency_name is '', the dependency is always not found.  So with
 `required: false`, this always returns a dependency object for which the