Document variable naming conventions in "conventions"
Document variable naming conventions in "conventions"

There are a number of variable naming conventions in set.mm;
try to document them.

Signed-off-by: David A. Wheeler <9ae72d22d8b894b865a7a496af4fab6320e6abb2@dwheeler.com>

metamath/set.mm,metamath/set.mm,metamath/set.mm