fsck doc: remove ancient out-of-date diagnostics

fsck doc: remove ancient out-of-date diagnostics

Remove diagnostics that haven't been emitted by "fsck" or its
predecessors for around 15 years. This documentation was added in
c64b9b88605 (Reference documentation for the core git commands.,
2005-05-05), but was out-of-date quickly after that.

Notes on individual diagnostics:

 - "expect dangling commits": Added in bcee6fd8e71 (Make 'fsck' able
   to[...], 2005-04-13), documented in c64b9b88605. Not emitted since
   1024932f019 (fsck-cache: walk the 'refs' directory[...],
   2005-05-18).

 - "missing sha1 directory": Added in 20222118ae4 (Add first cut at
   "fsck-cache"[...], 2005-04-08), documented in c64b9b88605. Not
   emitted since 230f13225df (Create object subdirectories on demand,
   2005-10-08).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>