Improve documentation: async calls (#1286)

[ifrost]

Purpose (TL;DR) - mandatory

This is to improve documentation of async stub calls - http://sinonjs.org/releases/v4.1.5/stubs/#stubcallsargasyncindex

Background (Problem in detail) - optional

Issue: #1286.
It also fixes duplicated docs mentioned in #1618.
Updated for new and current release.

How to verify - mandatory

1. Check out this branch
2. npm install
3. cd docs
4. build docs: bundle exec jekyll serve

[mantoni]

LGTM

[mroderick]

Thank for your contribution. I only have one comment, which is to reduce the indentation of the bullet lists, as first level bullets in Markdown don't need indenting.

    * not this
* but this

[ifrost]

Thank for your contribution. I only have one comment, which is to reduce the indentation of the bullet lists, as first level bullets in Markdown don't need indenting.

Sure thing! Updated.

[mroderick]

Thank you 👍