diff-tree doc: correct & remove wrong documentation

diff-tree doc: correct & remove wrong documentation

The documentation saying that diff-tree didn't support anything except
literal prefixes hasn't been true since
d38f28093e ("tree_entry_interesting(): support wildcard matching",
2010-12-15), but this documentation was not updated at the time.

Since this command uses pathspecs like most other commands, there's no
need to show examples of how the various "cmd <revs> <paths>"
invocations work.

Furthermore, the "git diff-tree --abbrev 5319e4" example shown here
never worked. We'd ended up with that through a combination of
62b42d3487 ("docs: fix some antique example output", 2011-05-26) and
ac4e086929 ("Adjust core-git documentation to more recent Linus GIT.",
2005-05-05), but "git diff-tree <tree>" was always invalid.

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