Improve composite control documentation

[flovogt]

Chapter https://ui5.sap.com/#/topic/d6bab27b5dc041b29b419bae8ae8f1d1 is empty. We should describe better whats the difference between Composite and XML Composite controls and when to use what.

[flovogt]

Also the usage of a JSONModel to maintain the state of a composite control could be described.

[margopolo]

As XMLComposites are deprecated, I don't think we should start to document them now?

[flovogt]

In general we should describe better

• How "JS Composites" can be used?
• When "JS Composites" shoudl be used and in which cases a real custom control should be written?
• Regarding XML Composites: Its still present in the 1.X version and people should be informed that this module is deprecated and explain how to migrate an existing XML Composite to a JS Composite

[boghyon]

To add: description that internally aggregated controls are not always available, as this.getAggregation("_myControl") can return null or undefined.

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