fix docstring of AddWithLimits
Merge pull request #2968 from zanzibar7/AddWithLimits_docstring

fix docstring of AddWithLimits
add nonpositive/nonnegative handling to Pow
fix test of evalf in facts
add I**im handling to assumptions
more general forms of tests and exp(i) and log(exp(i)) handling
add additional handling to old assumptions
add symbolic mod handling so Mod(4*integer, 4) == 0
rework assumptions watching for 0