drm_hwcomposer: Add links to the HWC2 API description

Some part of the API is not streightforward and wrong implementation
may cause leaks and other hard-to-debug runtime issues.

Signed-off-by: Roman Stratiienko <r.stratiienko@gmail.com>
diff --git a/DrmHwcTwo.h b/DrmHwcTwo.h
index 748fc81..111c664 100644
--- a/DrmHwcTwo.h
+++ b/DrmHwcTwo.h
@@ -119,6 +119,12 @@
 
     UniqueFd acquire_fence_;
 
+    /*
+     * Release fence is not used.
+     * There is no release fence support available in the DRM/KMS. In case no
+     * release fence provided application will use this buffer for writing when
+     * the next frame present fence is signaled.
+     */
     UniqueFd release_fence_;
 
    private: