config: document previously undocumented constants

Library/Homebrew/config.rb

+# Where downloads (bottles, source tarballs, etc.) are cached
 HOMEBREW_CACHE = Pathname.new(ENV["HOMEBREW_CACHE"] || "~/Library/Caches/Homebrew").expand_path
 
 # Where brews installed via URL are cached
 HOMEBREW_CACHE_FORMULA = HOMEBREW_CACHE/"Formula"
 
 if ENV["HOMEBREW_BREW_FILE"]
+  # Path to `bin/brew` main executable in {HOMEBREW_PREFIX}
   HOMEBREW_BREW_FILE = Pathname.new(ENV["HOMEBREW_BREW_FILE"])
 else
   odie "HOMEBREW_BREW_FILE was not exported! Please call bin/brew directly!"
@@ -15,7 +17,10 @@ HOMEBREW_PREFIX = Pathname.new(ENV["HOMEBREW_PREFIX"])
 # Where .git is found
 HOMEBREW_REPOSITORY = Pathname.new(ENV["HOMEBREW_REPOSITORY"])
 
+# Where we store most of Homebrew, taps, and various metadata
 HOMEBREW_LIBRARY = Pathname.new(ENV["HOMEBREW_LIBRARY"])
+
+# Where wrapper scripts for Git, Subversion, and various build tools are stored
 HOMEBREW_ENV_PATH = HOMEBREW_LIBRARY/"ENV"
 
 # Where we store lock files
@@ -24,13 +29,16 @@ HOMEBREW_LOCK_DIR = HOMEBREW_LIBRARY/"Locks"
 # Where we store built products
 HOMEBREW_CELLAR = Pathname.new(ENV["HOMEBREW_CELLAR"])
 
+# Where build, postinstall, and test logs of formulae are written to
 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
+  # Root of the Homebrew code base
   HOMEBREW_LIBRARY_PATH = Pathname.new(__FILE__).realpath.parent
 end
 
+# Load path used by standalone scripts to access the Homebrew code base
 HOMEBREW_LOAD_PATH = HOMEBREW_LIBRARY_PATH