Docs: Document <LangVersion> for csproj files

[BillWagner]

The C# build system now includes a node inside a csproj file that controls which version of C# the compiler should follow. Valid values include:

• ISO-1
• ISO-2
• C# 3.0
• C# 4.0
• C# 5.0
• C# 6.0
• C# 7.0
• C# 7.1 (beginning with .NET Core 2.0)
• latest (latest minor version)
• default (latest major version)

This can be set using the Visual Studio IDE, or editing the CSProj file.

@mairaw Can this be done with a dotnet CLI command?

[grokky1]

@BillWagner Also please document:

Visual Studio has an option for language version with "C# latest major version (default)". Which I assume means "7.0" rather than "7.1" which is confusing because most advice in SO/blogs is to do nothing and get the latest and greatest automatically though msbuild defaults.

But actually, to get 7.1 I need to do <LangVersion>latest</LangVersion>.

[mairaw]

Might be done as part of #2642. The new topic for 7.1 already mentions this.

[mairaw]

Also re. your question:

Can this be done with a dotnet CLI command?

I haven't seen that. Adding @livarcocc and @nguerrera to help with that.

[BillWagner]

One issue is to point out that you should select "all configurations", not just "debug" (which is the default).
This is a common pitfall, from the feedback so far.
So it would be great if we could include that in future instructions.