check-stagedir.sh and %%PORT{DOCS,EXAMPLES}%%

[Raphael Kubo da Costa]

The usual advice for ports that support staging is to remove the checks
for ${PORT_OPTIONS:MDOCS} and friends and unconditionally create
${STAGEDIR}${DOCSDIR} and copy files there (the same applies to
MEXAMPLES and EXAMPLESDIR).

This seems to work fine in the default case (ie. DOCS and EXAMPLES are
set in the port's configuration). If I take a simple port such as
textproc/chpp, unset any of those two options and run something like
`make DISTDIR=/tmp clean stage check-orphans`, check-stagedir.sh
complains about missing plist entries protected by
%%PORT{DOCS,EXAMPLES}%% even though they are actually present.

Other ports just set PORTDOCS and PORTEXAMPLES in the Makefile, and the
same thing happens.

Is this a bug in the script or is there something obvious I'm missing?