[INTERNAL] sap.ui.model.Context: Improve class documentation.
[INTERNAL] sap.ui.model.Context: Improve class documentation.

PS1: Improve documentation
PS2: Review comments GKL

Change-Id: I3d8cbdfb422df4456860d214496274406a937fd7
demo-kit-feedback: 3748

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