Rework text to introduce definition of CCS.

CCS ultimately wasn't mentioned until much later in the GPLv3 sections,
where, ironically, we have to point out that GPLv3 defined the term as
"Corresponding Source" [0], not CCS, and explain why GPL enforcement
wonks still say CCS.

This rework now introduces the acronym at the natural moment: while
describing GPLv2§3's use of the words "complete" and "corresponding".

Adding that made the section even more disjoint than it already was. I
put in some \subsection's to make it slightly less so, and did some
wordsmith work on surrounding text.

[0] I wish some GPLv3 drafter had asked me what to call the defined term
so that I could point out what fit standard parlance. :)