[gentoo-pms] Clarify line continuation in make.defaults

[Jacob Godserv]

This patch fixes a fairly small issue, but one that "got" me enough
times, while writing a PMS-compatible tool, to make me want to patch.

The wording on how a quoted string might continue over more than one
line is only barely enough to grasp its meaning. It requires some deep
logical thought to figure out what's actually meant here, so I've
added a little bit of extra explanation.

I found that nano set to COLUMNS=107 produced word wrapping similar to
the rest of the file, but let me know if I missed other formatting
rules I ought to know about.

-- 
    Jacob

    "For then there will be great distress, unequaled
    from the beginning of the world until now — and never
    to be equaled again. If those days had not been cut
    short, no one would survive, but for the sake of the
    elect those days will be shortened."

    Are you ready?