[MathML3-last-call] Clarify how a missing divisor or result are given for mlongdiv

The spec doesn't say how to specify a missing divisor or result for
mlongdiv.  For other mstack entries, <none> can be given for missing digits
and even a missing row (not sure why that would be useful), but it is silent
for these two (special) cases.

I have modified the beginning of the second paragraph in 3.6.2.1 (long
division description) to be:
"The result or divisor may be an elementary math element or may be none."

/w3ccvs/WWW/Math/Group/spec/xml/presentation-markup.xml,v  <--
presentation-markup.xml
new revision: 1.290; previous revision: 1.289