Document special character escaping in property files

[jacodg]

Make Frank!Developers aware of the fact that the Frank!Framework is using the standard Java classes to read property files which means special characters need to be escaped. For more information see:

frankframework/frankframework#5590
https://docs.oracle.com/cd/E23095_01/Platform.93/ATGProgGuide/html/s0204propertiesfileformat01.html
https://frank-manual.readthedocs.io/en/latest/advancedDevelopment/properties/properties.html?highlight=properties

[nielsm5]

Also please document this on the page where you explain the credentials.properties:
https://frank-manual.readthedocs.io/en/latest/advancedDevelopment/authorization/secrets.html