Add nullable type annotations and improve documentation for Nullness module

Author: TheAngryByrd

src/FsToolkit.ErrorHandling/Nullness.fs

@@ -2,48 +2,56 @@ namespace FsToolkit.ErrorHandling
 
 open System
 
+/// Represents errors that occur during application execution. In F# 9.0 and later, this type is marked as nullable.
 type ExceptionNull =
 #if NET9_0_OR_GREATER && !FABLE_COMPILER
     Exception | null
 #else
     Exception
 #endif
 
+/// Provides a mechanism for releasing unmanaged resources. In F# 9.0 and later, this type is marked as nullable.
 type IDisposableNull =
 #if NET9_0_OR_GREATER && !FABLE_COMPILER
     IDisposable | null
 #else
     IDisposable
 #endif
 
+/// Provides a mechanism for releasing unmanaged resources asynchronously. In F# 9.0 and later, this type is marked as nullable.
 type IAsyncDisposableNull =
 #if NET9_0_OR_GREATER && !FABLE_COMPILER
     IAsyncDisposable | null
 #else
     IAsyncDisposable
 #endif
 
+/// An abbreviation for the CLI type System.Object. In F# 9.0 and later, this type is marked as nullable.
 type ObjNull =
 #if NET9_0_OR_GREATER && !FABLE_COMPILER
     objnull
 #else
     obj
 #endif
 
+
+/// An abbreviation for the CLI type System.Collections.Generic.IEnumerable. In F# 9.0 and later, this type is marked as nullable.
 type SeqNull<'T> =
 #if NET9_0_OR_GREATER && !FABLE_COMPILER
     seq<'T> | null
 #else
     seq<'T>
 #endif
 
+/// An abbreviation for the CLI type System.String. In F# 9.0 and later, this type is marked as nullable.
 type StringNull =
 #if NET9_0_OR_GREATER && !FABLE_COMPILER
     string | null
 #else
     string
 #endif
 
+/// An abbreviation for the CLI type System.Collections.Generic.List`1 . In F# 9.0 and later, this type is marked as nullable.
 type ResizeArrayNull<'T> =
 #if NET9_0_OR_GREATER && !FABLE_COMPILER
     ResizeArray<'T> | null
@@ -55,8 +63,10 @@ type ResizeArrayNull<'T> =
 module internal Nullness =
 
 #if NET9_0_OR_GREATER && !FABLE_COMPILER
+    /// Throw a System.ArgumentNullException if the given value is null exceptio
     let inline nullArgCheck name value = nullArgCheck name value
 #else
+    /// Throw a System.ArgumentNullException if the given value is null exceptio
     let inline nullArgCheck (name: string) (value: 'T) : 'T =
         if isNull value then
             raise (ArgumentNullException(name))