graft: correct documentation about options can be reapplied
Now options --user and --date can be reapplied after we hit a conflict.
Differential Revision: https://phab.mercurial-scm.org/D3678
## Avoid interference from actual test env:unset HGTEST_JOBSunset HGTEST_TIMEOUTunset HGTEST_PORTunset HGTEST_SHELL