Updated C# and .NET docs supplement (markdown)

C#-and-.NET-docs-supplement.md

@@ -31,7 +31,7 @@ You can only have 1 `default` branch, but `case _ when ...:` doesn't work. Howev
 
 ## MSBuild `Condition` placement
 
-On (older versions of?) VS, `Condition` is ignored if placed on a property. Create a new `<PropertyGroup/>`.
+On (older versions of?) VS, `Condition` is ignored if placed on a property/item. Create a new `<PropertyGroup/>`.
 
 ## MSBuild property evaluation execution order
 
@@ -49,6 +49,13 @@ Use `MSBuildProjectDirectory`. Note that the latter doesn't include a trailing s
 .NET will happily include NUL (`(char) 0`) in a string if you use `String..ctor(char[])`.
 WinForms' `Label.Text` stops reading at the first NUL for measurement/rendering, at least under Mono.
 
+## Preprocessor TFM constants and .NET Standard
+
+The table [here](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/preprocessor-directives#conditional-compilation) is good for reference, but mind the note hidden at the bottom:
+> The `NETSTANDARD<x>_<y>_OR_GREATER` symbols are only defined for .NET Standard targets, and not for targets that implement .NET Standard [...]
+
+That is, you must use `#if !(NETSTANDARD2_1_OR_GREATER || NETCOREAPP2_1_OR_GREATER)` and not just `#if !NETSTANDARD2_1_OR_GREATER`.
+
 ## `String.GetHashCode` stability
 
 The `GetHashCode` implementation for strings does not reflect the string's contents, and as such, the hash not stable between program instances.