Add note to use no args explicitly with `not_to`.

Pass the first non-RSpec lib call site instead of defaulting to the 3rd
site, due to the deeper callstack from the block form of `expect`.
Remove empty parens.

Stylistically, rubyists generally don't use them.
Merge pull request #318 from rspec/pr-288-for-2-99

Pr 288 for 2 99