fix: add description attribute to access control policy role

[landerss1]

Related issue

This PR fixes issue #213.

Proposed changes

As discussed with @aeneasr, the fix just adds the description attribute to the Role struct. Existing tests have been updated to include the new attribute, as well as the API documentation.

Checklist

• I have read the contributing guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security. vulnerability, I
  confirm that I got green light (please contact
  security@ory.sh) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further comments

Running the tests with$ go test ./... (as described in the contributing guidelines) had some issues, which I don't know how to resolve. However, the issues seems to have no relation to my changes. These are the output of relevance from the above command:

2020/06/16 22:41:43 Could not connect to database: Could not start resource: : Post "http://localhost:2375/images/create?fromImage=postgres&tag=9.6": dial tcp [::1]:2375: connectex: No connection could be made because the target machine a
ctively refused it.
2020/06/16 22:41:43 Could not connect to database: Could not start resource: : Post "http://localhost:2375/images/create?fromImage=mysql&tag=5.7": dial tcp [::1]:2375: connectex: No connection could be made because the target machine acti
vely refused it.

The tests were run in a Cygwin64 terminal on Windows, and the host had Docker installed.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

Lars Andersson seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[landerss1]

@aeneasr I signed the CLA online, but it's not updating the status. Still in pending.

[aeneasr]

Awesome, thank you! Don't worry about the CLA, I can see that it is signed but it sometimes has problems when the user names don't match :)