docs: prefer the awk+sh TAP driver over the perl one

docs: prefer the awk+sh TAP driver over the perl one

We had completed our shell+awk implementation of the TAP driver
months ago, but never documented it in the manual, continuing to
document only the "prototype" written in perl instead.  Time to
fix this, before the 1.12 release.

* doc/automake.texi (Use TAP with the Automake test harness): Now
we document the use of the awk+shell implementation of our TAP
driver, rather than of the perl one.
* tests/tap-doc2.test: Adjust to use the shell+awk implementation
of the TAP driver.