"log --pretty" documentation: do not forget "tformat:"
We forgot to list "tformat:<string>" when enumerating possible
values that "--pretty=<format>" can take. It was not described
that "--pretty='string with %s placeholder'" that is not understood
is DWIMmed as "--pretty=tformat:<that string>".
Further, it was unclear what "When omitted, defaults to 'medium'"
was meant. Is it "When --pretty=<something> was not given at all",
or is it "When --pretty is given without =<something>"? Clarify
that it is the latter.
Signed-off-by: Junio C Hamano <gitster@pobox.com>