Clarify docs for Nuget private packages

[jamietanna]

Tell us more.

Taking a look (as someone unfamiliar with Nuget) I'm personally unsure if our docs for private packages are correct:

• https://docs.renovatebot.com/getting-started/private-packages/#nuget notes a command that I don't see executed
• https://docs.renovatebot.com/nuget/#alternate-feeds should be linked to clarify that there's a requirement to add this to Renovate config too