Document .bkp files

Backpack testing uses .bkp files, which are slurped in when GHC is passed the -backpack flag. From some conversations I've had, this flag is meant only for internal use, in our testsuite.

Yet this aspect of GHC seems entirely undocumented. .bkp files have a concrete syntax implemented in the parser, a potentially user-accessible flag, and quite a bit of code to support it all... but with no description that I have seen. (Maybe I've just looked in the wrong place.)

This ticket is therefore to track the documentation of this feature. Generally, I would expect at least a passing mention in the manual ("GHC has this -backpack flag, meant for internal use only; not a user-facing feature") and a long Note describing how the files are used in practice in the testsuite and how they are implemented.

(This was born from a discussion in !1767 (closed), but no need to look there to understand this ticket.)