[css-backgrounds][css-position-3][css-transforms][css-text-3] Adjust value descriptions to reflect combined <*-percentage> values

[SebastianZ]

In #263, #264 and #269 I've changed percentage combination values to use the combined <*-percentage> values defined in V&U 3, though I've not changed the value descriptions accordingly.

The descriptions of the values should be merged like it's already done in CSS Fonts 3 (not published yet) and CSS Images 3.

Sebastian

[fantasai]

Fixed.

[fantasai]

And fixed the follow-up errors in the commit, thanks. :)

[SebastianZ]

Thank you for fixing this! 😄

Sebastian