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.)