SourceLink will not set InformationalVersion from PackageVersion

The guidance says

> If you're using Source Link, this version will be set on build with the NuGet package version plus a source control version.

This is not true, i.e. if you don't set `InformationalVersion` it will end up as `1.0.0+[sha1]` even if you set `PackageVersion` to `4.3.2`. Perhaps the sentence should read: "If you're using Source Link, a source control version will be appended on build."

This also means this guidance is misleading:

> ❌ AVOID setting the assembly informational version yourself.
> > Allow SourceLink to automatically generate the version containing NuGet and source control metadata.

It should perhaps instead read:

> ✔️ DO set the `InformationalVersion` to the same value as `PackageVersion`.
> > Allow SourceLink to automatically append the source control version.

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* ID: ffa1275e-9f51-c83e-7cbd-9dfc8abb6dea
* Version Independent ID: 899c196e-4aad-515f-2d04-525719244c69
* Content: [Versioning and .NET libraries](https://docs.microsoft.com/en-us/dotnet/standard/library-guidance/versioning#assembly-informational-version)
* Content Source: [docs/standard/library-guidance/versioning.md](https://github.com/dotnet/docs/blob/master/docs/standard/library-guidance/versioning.md)
* Product: **dotnet-fundamentals**
* GitHub Login: @JamesNK
* Microsoft Alias: **jamesnk**