Disambiguate TargetFrameworkVersion and TargetFamework in help

(moved from dotnet/cli#5921)

The command line help suggests:

```cmd
:: v1.6 is .NET Standard TFM
dotnet pack /p:TargetFrameworkVersion=v1.6 .
```

should work, however it errors out. Whereas `TargetFramework` switch does work:

```cmd
dotnet pack /p:TargetFramework=v1.6 .
```

The ask is to clarify the difference in help (perhaps with a core-specific example):

```cmd
dotnet msbuild -h
[snip]
MSBuild MyApp.csproj /t:Clean
/p:Configuration=Debug;TargetFrameworkVersion=v3.5
:: ^^^^^^^^^^ this one
```