hooklib: update documentation of changeset_obsoletedfor for changed hook type
This updates usage example of changeset_obsoleted to reflect the move from
pretxnclose to txnclose made in
04ef381000a8 (hooklib: fix detection of
successors for changeset_obsoleted).
Differential Revision: https://phab.mercurial-scm.org/D8929
local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed
slicing test passed
issnapshot test passed
findsnapshot test passed