Improve formatting in the hooks documentation

The pyeval role generates literal (e.g. ``OP_CLUSTER…``) formatting,
which differs from the other headers. This is confusing, so let's add
a note that these are virtual operations, which makes the output more
consistent.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>