Add information about escape syntax in documentation

[jkrzemin]

Describe the bug/feature In some cases, there is a need to use syntax similar to reclass reference interpolation - i.e. bash scripts in config maps.

Reclass supports this feature with \ character which is not mentioned in the docs.

To Reproduce create a bash script inside any resource that within the value would contain variable reference.

echo "My hostname is ${HOSTNAME}"

This will trigger an attempt to lookup parameters.HOSTNAME which we want to avoid.

Expected behavior In documentation there should be a mention of escape mechanism currently supported in reclass.

echo "My hostname is \${HOSTNAME}