Clarify the location of the Windows-specific ProgramData config

On Windows, there is no (single) `/etc/` directory. To address that, in
conjunction with the libgit2 project, Git for Windows introduced yet
another level of system-wide config files, located in C:\ProgramData
(and the equivalent on Windows XP).

Let's spell this out in the documentation.

This closes git-for-windows#470 (because
there was no reaction in three months in that Pull Request).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

Documentation/git-config.txt

@@ -247,9 +247,12 @@ $GIT_DIR/config::
 	Repository specific configuration file.
 
 On Windows, as there is no central `/etc/` directory, there is yet another
-config file, intended to contain settings for *all* Git-related software
-running on the machine. Consequently, this config file takes an even lower
-precedence than the `$(prefix)/etc/gitconfig` file.
+config file (located at `$PROGRAMDATA/Git/config`), intended to contain
+settings for *all* Git-related software running on the machine. Consequently,
+this config file takes an even lower precedence than the
+`$(prefix)/etc/gitconfig` file. Typically `$PROGRAMDATA` points to
+`C:\ProgramData` (on Windows XP the equivalent in `$ALLUSERSPROFILE` is used,
+i.e. `C:\Documents and Settings\All Users\Application Data\Git\config`).
 
 If no further options are given, all reading options will read all of these
 files that are available. If the global or the system-wide configuration