App: update to tooltip on env vars substitution

[konstan]

The tooltip doesn't work in all cases and hence, is misleading. It speaks about sending variables to services of stack, which is not suitable for the case below.

I suggest changing it to:

For Kubernetes manifest

To use variables from Configuration->"Environment Variables", reference them in 
the application definition below by e.g. ${MY-ENV-VAR}. Consult environment variables definition and 
substitution for Kubernetes manifest.

For Docker compose

To use variables from Configuration->"Environment Variables", reference them in 
the application definition below by e.g. ${MY-ENV-VAR}. Consult environment variables definition and 
substitution for Docker compose.