docs: brush up obsolete bits of git-fsck manpage

After the description and options, the fsck manpage contains
some discussion about what it does. Over time, this
discussion has become somewhat obsolete, both in content and
formatting. In particular:

  1. There are many options now, so starting the discussion
     with "It tests..." makes it unclear whether we are
     talking about the last option, or about the tool in
     general. Let's start a new "discussion" section and
     make our antecedent more clear.

  2. It gave an example for --unreachable using for-each-ref
     to mention all of the heads, saying that it will do "a
     _lot_ of verification". This is hopelessly out-of-date,
     as giving no arguments will check much more (reflogs,
     the index, non-head refs).

  3. It goes on to mention tests "to be added" (like tree
     object sorting). We now have these tests.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>