Guide: mention using -append/-delete in 4.2 Creating a Portfile

[ryandesign (Ryan Carsten Schmidt)]

Jim Busser sent this Guide feedback to macports-mgr:

Under 4.2 Creating a Portfile > 14. Port dependencies

where it says
	A port's dependencies are ports that must be installed before another port is installed.
and gives the example
	depends_lib
might it be worth cautioning to
	see 5.3.2.1 (bullet 2) on preferred use of the -append, -delete keyword modifiers

This should be considered not only for the depends_lib example but also for the configure.args example and the categories example, since portgroups commonly define those.