libcamera: base: class: Document Extensible::_d() functions

The Extensible::_d() functions are meant to be called by users of the
class. Document them.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>

src/libcamera/base/class.cpp

@@ -150,6 +150,23 @@ Extensible::Extensible(Extensible::Private *d)
 {
 }
 
+/**
+ * \fn Extensible::_d() const
+ * \brief Retrieve the private data instance
+ *
+ * This template function isn't meant to be called directly. Instead, classes
+ * derived from Extensible get, through the LIBCAMERA_DECLARE_PRIVATE() macro,
+ * overriden _d() functions that return the correct pointer type to the
+ * corresponding derived Private class.
+ *
+ * \return A pointer to the private data instance
+ */
+
+/**
+ * \fn Extensible::_d()
+ * \copydoc Extensible::_d() const
+ */
+
 /**
  * \var Extensible::d_
  * \brief Pointer to the private data instance