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
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sorting in roots_quadratic should be same as in Poly.all_roots(radicals=False)

fixes #8255
Mul._eval_is_positive call first Expr's helper to handle numbers
Fix tests after 58c61b2
Revert "sorting in roots_quadratic should be same as in Poly.all_roots(radicals=False)"

This reverts commit 58c61b242add404f3fcd078a3cce7321320acaf8.

Conflicts:
	sympy/polys/polyroots.py
sort roots in _roots_trivial that come from roots_quadratic

In polyroot, sorting is done with the default_sort_key.
RootOf instances (which don't have an explicit expression
to sort) are sorted on the basis of their intervals.
Since _roots_trivial uses roots_quadratic (from polyroots)
the roots have to be resorted to match the RootOf convention.
add test to confirm that radicals flag does not change root order
mcg