Added javadoc to cz.xtf.core.waiting.SimpleWaiter

[honza-kasik]

Added JavaDoc to cz.xtf.core.waiting.SimpleWaiter#SimpleWaite

Please make sure your PR meets the following requirements:

• [x] Pull Request contains a description of the changes
• [x] Pull Request does not include fixes for multiple issues/topics
• [x] Code is formatted, imports ordered, code compiles and tests are passing
• [x] Code is self-descriptive and/or documented

[honza-kasik]

@mchoma Can you please review or assign a reviewer?