Prolog:

[22]    prolog     ::=      XMLDecl? Misc* (doctypedecl Misc*)?
[27]    Misc       ::=      Comment | PI | S

What type of content, and where more specifically you expect it to be allowed?

In any case, SAX is just an API specification. The actual behavior is governed by:

  • The XML specification; and
  • The parser/processor implementation in use.