Add long_description_content_type to metadata table

docs/setuptools.txt

@@ -2394,9 +2394,9 @@ Metadata
     Aliases given below are supported for compatibility reasons,
     but not advised.
 
-=================  =================  =====
+==============================  =================  =====
 Key                             Aliases            Accepted value type
-=================  =================  =====
+==============================  =================  =====
 name                                               str
 version                                            attr:, str
 url                             home-page          str
@@ -2409,12 +2409,13 @@ classifiers        classifier         file:, list-comma
 license                                            file:, str
 description                     summary            file:, str
 long_description                long-description   file:, str
+long_description_content_type                      str
 keywords                                           list-comma
 platforms                       platform           list-comma
 provides                                           list-comma
 requires                                           list-comma
 obsoletes                                          list-comma
-=================  =================  =====
+==============================  =================  =====
 
 .. note::