help: add documentation for new template functions

changectx: fix the handling of `tip` We can not use `len(repo,changelog)`, it may be a filtered revision. We now use `repo,changelog.tip()` to fetch this information. The `tip` command is also fixed and tested Thanks goes to Idan Kamara for the initial report.

bisect: use changelog for iteration With changelog filtering, we can not use xrange anymore. We have to use the changelog to do the iteration. This way, the changelog excludes filtered revision and we can safely use what we iterate over. Without this changes, bisect crash with a traceback if there is filtered revision in the repo. Tests have been updated.