PR28902, -T script with INSERT ordering

The answer to PR28902 may be deduced from the existing INSERT
documentation that says the default script is parsed after the -T
INSERT script, if you assume (correctly) that nothing special is done
when inserting into -T scripts overriding the default script.  In both
cases INSERT handling looks for the specified output section later on
the internal list of parsed script commands.  This isn't obvious
though, so make the ordering explicit, and mention that section
assignments are the same too.

	PR 28902
	* ld.texi (INSERT): Specify ordering when -T is used both to
	override the default script and to augment.