Update comment in DefaultSettings for bug 17180

When a check to disable $wgShowIPinHeader if $wgUseFileCache is
enabled was added to Setup.php (bug 17180), the inline comment
documentation in DefaultSettings.php was not adjusted, and continues
to indicate that users must manually coordinate the values of
$wgShowIPinHeader and $wgUseFileCache.  This patch adjusts the comment
to reflect the current automatic coordination.

Bug: 55723
Change-Id: I8078a3847c9ef2f386702bca5b5272028dc41e31
Merge "Update comment in DefaultSettings for bug 17180"