docs: improve, extend and fix documentation on TAP support

docs: improve, extend and fix documentation on TAP support

* doc/automake.texi ("Using the TAP test protocol"): Divide this
section into ...
("Introduction to TAP", "Use TAP with the Automake test harness",
"Incompatibilities with other TAP parsers and drivers", "Links
and external resources"): ... these subsections, extend them by
adding more information and examples, and improve them by removing
incomplete and/or temporary wordings and TODO items.
("Script-based Testsuites", "Parallel Test Harness"): Add a couple
of anchors to improve the granularity of cross-references.
* tests/tap-doc2.test: New test, verifying the correctness of the
new examples given in the manual.
* tests/Makefile.am (tap_other_tests): Add the new test.