[INTERNAL] core.Icon: improve the documentataion of color properties
[INTERNAL] core.Icon: improve the documentataion of color properties

- it wasn't clear which value can be used to set the color properties.

- this change add additional documentation to state explicitly that
  either sap.ui.core.CSSColor or sap.ui.core.IconColor can be used.

Change-Id: I640136b87358165998544e6803b4fee662574613

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