Improve documentation around Helm subchart values

[michaelsauter]

Currently it may be read such that all values must be set from the umbrella repo, when in fact values.yaml files in subcharts are respected.

FYI @henrjk

[henrjk]

This sentence mislead me: "Note that values and secrets files are only collected from the repository for which the pipeline runs. " in file https://github.com/opendevstack/ods-pipeline/blob/01a67e7a6a1dcb98045e73bbefc939cb854084c9/docs/tasks/ods-deploy-helm.adoc

[michaelsauter]

Transferring as a consequence of opendevstack/ods-pipeline#722.