Inconsistency between Install Scenarios in package reference and packages.config cases clarifications

[nkolev92]

We need to clarify the different behavior between install in package reference (both netcore and legacy) cases and packages.config.

In the packages.config case installing with a primary source selected means that the primary source has to contain the requested package and the dependencies can come from any other of the not-selected, but enabled sources.

In package reference case, all sources are used and the dropdown is just a view.
The concept of primary source is not relevant since the install is treated like a regular restore.
This is by design because different sources in the transitive cases can lead to different resolution graphs, thus leading to inconsistent restores.

We need to document both of these scenarios, as users have already reported with concern about the behavior.

Example this issue:
https://developercommunity.visualstudio.com/content/problem/69961/manage-nuget-packages-for-solution-is-not-paying-a.html

Related to #5763
Related to #5863 as well.
Maybe related to #5932 [closed]
Related to #5643
Related to https://github.com/NuGet/docs.microsoft.com-nuget/issues/407
//cc
@emgarten @anangaur @rrelyea