Add documentation on usage of Camel simple expression in configuring property

[gansheer]

Description

The documentation lack some explanation on how to use a Camel Simple Expression in a configuration property in the application.properties file.

Adding a section in the user guide to using a config property for a Camel simple expression and explaining that expressions like ${body} need special treatment (E.g using the raw value) would be very helpful.

As an example:

• this is a valid expression embedded as a parameter in the route: expression: "${body} contains 'not_in'"
• this is the same valid expression as a property: myexpression=\\${body} contains 'not_in'

Note: this is a follow up of this discussion on zulip.