docs: improve Helm README (existing postgresql Secret) (#3991)
docs: improve Helm README (existing postgresql Secret) (#3991)

Small doc improvements:
- Add the 2 missing parameters postgresql.existingSecret and postgresql.existingSecretKey in the list.
- Update the "Using an external PostgreSQL server" part by specifying how to use an existing postgresql Secret

***NO_CI***
Requarks/wiki,Requarks/wiki,Requarks/wiki,Requarks/wiki