Specify that explicit ids are asciidoctor only

[MasterOdin]

asciidoc-py does not support explicit ids and attempting to use them causes errors on parsing. For example, given the following file:

= Hello

[#primitives-nulls]
== Primitive types and null values

gives the following when attempted to be translated by asciidoc-py:

$ python3 ../asciidoc-py3/asciidoc.py test.asciidoc
asciidoc: WARNING: test.asciidoc: line 4: missing section: [#explicit-id]

and the resulting file does not have the second header at all. Asciidoctor handles this as documented, with the second header appearing and with an explicitly set id.

[mojavelinux]

Now that the AsciiDoc Language project is underway using AsciiDoc as defined by Asciidoctor as the starting point, I'm not going to sprinkle differences between AsciiDoc.py and Asciidoctor throughout this docs site any longer. Instead, that information should be relegated to the following migration page: https://docs.asciidoctor.org/asciidoctor/latest/migrate/asciidoc-py/ Feel free to submit a change to that page to call attention to it.