(various): document from_promisor parameter

(various): document from_promisor parameter

88e2f9ed8e ("introduce fetch-object: fetch one promisor object",
2017-12-05) plumbed through the from_promisor parameter but did
not document it everywhere it appeared. Add the documentation.

(It also plumbed through the no_dependents parameter, but I have left
that alone because it is being removed in a commit under review [1].)

[1] https://lore.kernel.org/git/e8f16d69089a5011c355d5939c56fa53b7a1eb2d.1597184949.git.jonathantanmy@google.com/

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>