[Doc-SIG] formalizing StructuredText

[Edward D. Loper]

I wanted to make sure my terminology was clear, because it looked
like the indentation got messed up somehow.  My terms are:
    * valid
    * invalid
        * illegal
        * undefined

I.e., "illegal" and "undefined" are mutually-exclusive, collectively
exhaustive *subsets* of "invalid."

We would use "illegal" for strings like::

  * This ** is * a ** very * bad ** string *

We would currently use "undefined" for strings like::

  * Nesting is not yet **implemented** *

Both are "invalid" for the current version of STpy, i.e., their meaning
is undefined.  But we *never* intend to give a meaning to the first
one.  Of course, an implementation can still give it a structure, if
the user asks it to..  But "illegal" strings will *never* be defined
under STminus.

Hm.. I hope that's clearer.. :)

-Edward