Integrate pasted explanation of GPLv2§2(c).

The tutorial did not previously have an adequate description of
GPLv2§2(c), so this pasted text is as good as any that I could think of,
although I reworded it slightly.

I chose to integrate this text *before* the discussion of GPLv2§2(b)
because the latter section requires so much attention, it seems
appropriate to get the less complex parts of GPLv2§2 out of the way
before diving into that essential discussion of GPLv2§2(b).