Add note about Version constructor.

Library/Homebrew/os/mac/version.rb

@@ -117,6 +117,7 @@ module OS
       alias requires_popcnt? requires_nehalem_cpu?
 
       # Represents the absence of a version.
+      # NOTE: Constructor needs to called with an arbitrary macOS-like version which is then set to `nil`.
       NULL = Version.new("10.0").tap { |v| v.instance_variable_set(:@version, nil) }.freeze
     end
   end

Library/Homebrew/version.rb

@@ -736,5 +736,6 @@ class Version
   end
 
   # Represents the absence of a version.
+  # NOTE: Constructor needs to called with an arbitrary non-empty version which is then set to `nil`.
   NULL = Version.new("NULL").tap { |v| v.instance_variable_set(:@version, nil) }.freeze
 end