doc: add caveat about turning off commit-graph

The doc `technical/commit-graph.adoc` says that replace objects and
commit grafts turn off commit-graph:

    Commit grafts and replace objects can change the shape of the commit
    history. The latter can also be enabled/disabled on the fly using
    `--no-replace-objects`. This leads to difficulty storing both possible
    interpretations of a commit id, especially when computing generation
    numbers. The commit-graph will not be read or written when
    replace-objects or grafts are present.

But this isn’t mentioned in the user-facing doc. Let’s mention it on
git-replace(1) and git-commit-graph(1).

Acked-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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