Dropping $txpcfg['doc_root'].

According to comments, this fellow really never existed.
It was deprecated a decade ago.

Even diagnostics panel doesn't do that 'IIS' normalization.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5437 2fea6a4d-a838-0410-917b-93a53c48e9d2
Document 'LANG' constant.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5438 2fea6a4d-a838-0410-917b-93a53c48e9d2