Add a section "Update Service Credentials" without downtime
Add a section "Update Service Credentials" without downtime

The current documentation about [Update Service Credentials](https://docs.cloudfoundry.org/devguide/services/application-binding.html#update-credentials) require
a downtime. Service credentials could be updated without a downtime also.
This commit extends the [Update Service Credentials](https://docs.cloudfoundry.org/devguide/services/application-binding.html#update-credentials) section with steps to update
service binding credentials without a downtime.

cloudfoundry/docs-dev-guide