sgml-tag-help

sgml-tag-help is a variable defined in `sgml-mode.el'.
Its value is

(("!" . "Empty declaration for comment")
 ("![" . "Embed declarations with parser directive")
 ("!attlist" . "Tag attributes declaration")
 ("!doctype" . "Document type (DTD) declaration")
 ("!element" . "Tag declaration")
 ("!entity" . "Entity (macro) declaration"))

Documentation:
Alist of tag name and short description.

You can customize this variable.