Move timex docs to the top of the file (#3351)

Move the docs for the supported `timex` exporter above the docs of the
deprecated `ntp` module. Many folks see the word "deprecated" and stop
reading without realizing there's a supported option buried at the
bottom of the file.

Signed-off-by: David M. Lee <leedm777@yahoo.com>
Co-authored-by: Ben Kochie <superq@gmail.com>