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 This is hopelessly out-of-date,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: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-fsck.txt

@@ -72,30 +72,20 @@ index file, all SHA1 references in .git/refs/*, and all reflogs (unless
 	a blob, the contents are written into the file, rather than
 	its object name.
 
-It tests SHA1 and general object sanity, and it does full tracking of
-the resulting reachability and everything else. It prints out any
-corruption it finds (missing or bad objects), and if you use the
-'--unreachable' flag it will also print out objects that exist but
-that aren't reachable from any of the specified head nodes.
-
-So for example
-
-	git fsck --unreachable HEAD \
-		$(git for-each-ref --format="%(objectname)" refs/heads)
+DISCUSSION
+----------
 
-will do quite a _lot_ of verification on the tree. There are a few
-extra validity tests to be added (make sure that tree objects are
-sorted properly etc), but on the whole if 'git fsck' is happy, you
-do have a valid tree.
+git-fsck tests SHA1 and general object sanity, and it does full tracking
+of the resulting reachability and everything else. It prints out any
+corruption it finds (missing or bad objects), and if you use the
+'--unreachable' flag it will also print out objects that exist but that
+aren't reachable from any of the specified head nodes (or the default
+set, as mentioned above).
 
 Any corrupt objects you will have to find in backups or other archives
 (i.e., you can just remove them and do an 'rsync' with some other site in
 the hopes that somebody else has the object you have corrupted).
 
-Of course, "valid tree" doesn't mean that it wasn't generated by some
-evil person, and the end result might be crap. git is a revision
-tracking system, not a quality assurance system ;)
-
 Extracted Diagnostics
 ---------------------