docs: fix unescaped html tag (#802)

If the description of the method contains any HTML tag, it will
break the HTML rendering.
This commit escapes the html tag to prevent this problem.
* Use html standard lib when using python3
* Use cgi standard lib when using python2

Refs #791

Release-As: 1.8.2

Author: Billy4195

describe.py

@@ -247,6 +247,12 @@ def method(name, doc):
   """
 
     params = method_params(doc)
+    if sys.version_info.major >= 3:
+        import html
+        doc = html.escape(doc)
+    else:
+        import cgi
+        doc = cgi.escape(doc)
     return string.Template(METHOD_TEMPLATE).substitute(
         name=name, params=params, doc=doc
     )