Update std.stdio.writef docs.

The docs are outdated; the `writef(stderr, fmt, ...)` syntax is no longer
supported. Also, remove a bunch of really old notes about changes in version
2.006, which are already ancient history. Refer instead to the docs of
std.format.formattedWrite, which are the most up-to-date and accurate docs on
this subject.
Merge pull request #2540 from quickfur/issue10139

Update std.stdio.writef docs.