Update guidance on ThrowIfNull optional parameter (dotnet#7971)

Author: gewarren

xml/System/ArgumentNullException.xml

@@ -403,9 +403,9 @@
  </Parameters>
  <Docs>
  <param name="argument">The reference type argument to validate as non-null.</param>
- <param name="paramName">The name of the parameter with which <paramref name="argument" /> corresponds.</param>
+ <param name="paramName">The name of the parameter with which <paramref name="argument" /> corresponds. If you omit this parameter, the name of <paramref name="argument" /> is used.</param>
  <summary>Throws an <see cref="T:System.ArgumentNullException" /> if <paramref name="argument" /> is <see langword="null" />.</summary>
- <remarks>To be added.</remarks>
+ <remarks>The <paramref name="paramName" /> parameter is included to support the <see cref="System.Runtime.CompilerServices.CallerArgumentExpressionAttribute" /> attribute. It's recommended that you don't pass a value for this parameter and let the name of <paramref name="argument" /> be used instead.</remarks>
  <exception cref="T:System.ArgumentNullException">
  <paramref name="argument" /> is <see langword="null" />.</exception>
  </Docs>