Extended documentation of ImageViewHelper.php

[chludwig]

Added more information to the documentation of width and height parameter in ImageViewHelper.php

[TYPO3IncTeam]

Thank you for your contribution to TYPO3. We are using Gerrit Code Review for our contributions and took the liberty to convert your pull request to a review in our review system.
You can find your patch at: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81717
For further information on how to contribute have a look at https://docs.typo3.org/typo3cms/ContributionWorkflowGuide/