config.rb: add comment explaining HOMEBREW_TEMP logic

Library/Homebrew/config.rb

@@ -45,6 +45,7 @@ HOMEBREW_CELLAR = Pathname.new(ENV["HOMEBREW_CELLAR"])
 
 HOMEBREW_LOGS = Pathname.new(ENV["HOMEBREW_LOGS"] || "~/Library/Logs/Homebrew/").expand_path
 
+# Must use /tmp instead of $TMPDIR because long paths break Unix domain sockets
 HOMEBREW_TEMP = Pathname.new(ENV.fetch("HOMEBREW_TEMP", "/tmp"))
 
 unless defined? HOMEBREW_LIBRARY_PATH