Document where configuration files are in config.txt

Talking about what the files contain without talking about where
they are does not help new users.

Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/config.txt

@@ -2,7 +2,12 @@ CONFIGURATION FILE
 ------------------
 
 The git configuration file contains a number of variables that affect
-the git command's behavior. They can be used by both the git plumbing
+the git command's behavior. `.git/config` file for each repository
+is used to store the information for that repository, and
+`$HOME/.gitconfig` is used to store per user information to give
+fallback values for `.git/config` file.
+
+They can be used by both the git plumbing
 and the porcelains. The variables are divided into sections, where
 in the fully qualified variable name the variable itself is the last
 dot-separated segment and the section name is everything before the last