[doc] use debug mode to build (#6151)

docs/README.md

@@ -17,6 +17,6 @@ Note:
 
 - If C++ codes have been changed, we suggest to remove the previous results to
   triggle the rebuild for all pages, namely run `make clean_docs`.
-
+- If C++ codes are failed to build, run `make clean`
 - If CSS or javascript are changed, we often need to do a *force refresh* in the
   browser to clear the cache.

docs/mxdoc.py

@@ -39,7 +39,7 @@ def generate_doxygen_xml(app):
 
 def build_mxnet(app):
     """Build mxnet .so lib"""
-    _run_cmd("cd %s/.. && cp make/config.mk config.mk && make -j$(nproc)" %
+    _run_cmd("cd %s/.. && cp make/config.mk config.mk && make -j$(nproc) DEBUG=1" %
             app.builder.srcdir)
 
 def build_r_docs(app):
@@ -267,7 +267,7 @@ def setup(app):
     app.connect("builder-inited", build_mxnet)
     # skipped to build c api doc
     # app.connect("builder-inited", generate_doxygen_xml)
-    # app.connect("builder-inited", build_scala_docs)
+    app.connect("builder-inited", build_scala_docs)
     # skipped to build r, it requires to install latex, which is kinds of too heavy
     # app.connect("builder-inited", build_r_docs)
     app.connect('source-read', convert_table)