Update & Extend documentation

[bossenti]

While working with the tableau server-client-python library I stumbled about some bugs in the documentation which I fixed within this PR.
Furthermore, I've created documentation for the method add_to_schedule() of the schedules-endpoint as this was missing so far.

Hopefully this PR fits so far, otherwise feel free to let me know.

Thanks for this great library, I really like the API of the client. It's fun to work with it :)

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @bossenti to sign the Salesforce.com Contributor License Agreement.

[bcantoni]

@bossenti thanks for the contribution here! I've added a few corrections to make but otherwise it's looking good.

[salesforce-cla]

Thanks for the contribution! Unfortunately we can't verify the commit author(s): Tim Bossenmaier <t***@c***.com>. One possible solution is to add that email to your GitHub account. Alternatively you can change your commits to another email and force push the change. After getting your commits associated with your GitHub account, sign the Salesforce.com Contributor License Agreement and this Pull Request will be revalidated.

[bossenti]

Thanks @kramantab and @bcantoni for reviewing.
I've updated the PR according to your comments.

[bcantoni]

All looks good to me. Passing it over to @kramantab for any additional feedback or changes.

@bossenti after @kramantab approves, you can merge. Please choose the "Squash and merge" option to combine all your commits into one.

[kramantab]

Looking good Thanks!

[bossenti]

@bcantoni, @kramantab, unfortunately I can't merge the pull request because I lack the appropriate permissions: Only those with write access to this repository can merge pull requests.

[bcantoni]

Done - thanks @bossenti for the contribution!