docs: some fixlets in documentation on testsuites support

docs: some fixlets in documentation on testsuites support

* doc/automake.texi (Script-based Testsuites): Rename node ...
(Scripts-based Testsuites): ... to this.  Break overly long lines
in an example (were causing problems with PDF output).
(Parallel Test Harness): Don't intend paragraphs that come after
examples or bulleted list.
(Overview of Custom Test Drivers Support): Fix typo s/the the/the/.
(Declaring Custom Test Drivers): When giving an aside, prefer comma
to parentheses, as it disrupts the reading flow less.
(API for Custom Test Drivers): Remove a sentence that was basically
duplicated from the previous subsection.
(Command-line arguments for test drivers): Don't intend paragraph
coming after an itemized list.  Clarify wording and fix grammaros.
Add a couple of cross-references to earlier explanations of `.log'
and `.trs' files.
(Links and external resources): Node renamed ...
(Links and external resources on TAP): ... to this, for clarity.
Slightly extend and clarify introductory sentence.