Document `-nullable:annotations` option (#39444)

docs/features/nullable-reference-types.md

@@ -6,10 +6,10 @@ Reference types may be nullable, non-nullable, or null-oblivious (abbreviated he
 
 Project level nullable context can be set by using "nullable" command line switch:
 -nullable[+|-]                        Specify nullable context option enable|disable.
--nullable:{enable|disable|warnings}   Specify nullable context option enable|disable|warnings.
+-nullable:{enable|disable|warnings|annotations}   Specify nullable context option enable|disable|warnings|annotations.
 
 Through msbuild the context could be set by supplying an argument for a "Nullable" parameter of Csc build task.
-Accepted values are "enable", "disable", "warnings", or null (for the default nullable context according to the compiler).
+Accepted values are "enable", "disable", "warnings", "annotations", or null (for the default nullable context according to the compiler).
 The Microsoft.CSharp.Core.targets passes value of msbuild property named "Nullable" for that parameter.
 
 Note that in previous preview releases of C# 8.0 this "Nullable" property was successively named "NullableReferenceTypes" then "NullableContextOptions".