[INTERNAL] sap.ui.core.CSSColor: Improve documentation regarding css color level restriction
[INTERNAL] sap.ui.core.CSSColor: Improve documentation regarding css color level restriction

Only values compliant with CSS Color Level 3 are supported.

Fixes: https://github.com/SAP/openui5/issues/3580
Change-Id: Ic967e0f0725c3b753d9e8947c0e6592acae37551

SAP/openui5,SAP/openui5,SAP/openui5,SAP/openui5