[CHANGE] Elements for writing about PreTeXt

[Rob Beezer]

The #attr, #tag, #tage elements are meant for use when documenting PreTeXt and
are meant just for the names themselves, rather than more elaborate bits of XML.
To wit, the documentation has long said "the content of these elements should
only be the 26 lower-case letters and a dash/hyphen."

The implementation has changed a bit to reflect this long-standing expecation
for the content of these elements. So if you have something more complicated,
you might get caught out. HTML may behave, and LaTeX might complain.

Paragraphs: <tag>, <tage>, <attr>, tag, empty tag, attribute, <section>,
<hash/>, @width
https://pretextbook.org/doc/guide/html/topic-paragraphs.html#topic-paragraphs-3-15