#501 Update MFTF documentation and Best Practices

[lbajsarowicz]

Description

Briefly updated the Documentation contents.

Fixed Issues (if relevant)

1. Feedback on page: /mftf/docs/test/action-groups.html #501

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/verification tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)
• Changes to Framework doesn't have backward incompatible changes for tests or have related Pull Request with fixes to tests

[coveralls]

Coverage increased (+1.5%) to 53.41% when pulling 4d91e49 on lbajsarowicz:bugfix/501-documentation into afcae2c on magento:develop.

[jilu1]

Approved by @dobooth