Docstring for Expr

fixes #4985
Merge pull request #8224 from skirpichev/expr-doc

Docstring for Expr
Binomial parameter checking
Merge pull request #8176 from leonidbl91/binomial_trails

Binomial parameter checking
Add leonidb to AUTHORS/aboutus. Welcome to SymPy!
show failing expression in nonzero
minor code tidy/modify
add filldedent to namespace
sort in _roots_trivial to match RootOf sorting

Also give roots_quadratic and roots_binomial a flag
to allow sorting to be skipped.

A robust numerical method is used for sorting since
complicated expressions arising from roots_binomial
sometimes fail comparison tests, raising the error
about symbolic expressions not having a truth value.
reverse roots in inequalities; add tests