docs: minor tweaks to sharing guide (suggested by timeless)

docs: add code review scenario to sharing guide The idea is to demonstrate a simpler multiple-developer situation that does not involve getting into trouble. The final scenario illustrates Alice and Bob getting into trouble with bumped and divergent changesets by amending each other's history. The required tests and text are all written, but will need to be heavily revised because of the inserted scenario.

docs: add a bit more hierarchy to the sharing guide

docs: update sharing guide based (mostly) on marmoute's review - don't claim certain scenarios are more/less common, just simple/advanced - mention code review as a multiple-developer scenario (not described in detail yet!) - suggest "hg config --edit --local" instead of "cat >> .hg/hgrc" - use -q less often (and show resulting output) - edit some section headers to be consistent with user guide (example numbers; "amend" instead of "amending") (These are just the small changes; big changes are yet to come.)