same symbol diff asdumptions makes two outputs in sym, findsymbols

Improve docs, tests
symfun: improve symvar

symvar skould prefer the explicit vars of the symfun, at least
for `symvar(f, n)` form.  `symvar(f)` still gives alphabetical
order.

Some discrepancy with SMT here: what to do when symfun does not
depend on its vars?  Left fixme and xtest for this.