update comments

paddle/scripts/docker/build.sh

@@ -44,8 +44,9 @@ fi
 make install
 pip install /usr/local/opt/paddle/share/wheels/*.whl
 
-# Since python v2 api import py_paddle module, the generation of paddle docs
-# depend on paddle's compilation and installation
+# To build documentation, we need to run cmake twice.
+# This awkwardness is due to https://github.com/PaddlePaddle/Paddle/issues/1854.
+# It also describes a solution.
 if [ ${WITH_DOC} == "ON" ]; then
     mkdir -p /paddle/build_doc
     pushd /paddle/build_doc