[Config] [Configuration] Emphasize env vars purpose

[alexandre-daubois]

Fix #14745

[94noni]

i like the paragraph mostly
small comments as well

[javiereguiluz]

Alex, this was a nice contribution as always.

However, while merging I notices that the new note explained a use case for env vars ... but in the intro of this section we mentioned another use case. So, I removed the note and merged its contents into the section intro to make it easier to readers. See 149876b