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 https://github.com/git-for-windows/git/pull/470 (because
there was no reaction in three months in that Pull Request).
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>