[IMP] doc: add some diagrams to workflow doc

Also move transitions to the bottom of the document, and try to clarify the
documentation for split, join and activity kinds.
[FIX] Remove duplicate "Can be purchased" filter.

Duplicate product filter "To Purchase" and "Can be Purchased". Same search
criteria and same name, so removing one doesn't affect people who were
referencing it (e.g. with search_default_* key in context).  Left the one
named "Can be Purchased" because consistent with the equivalent sale filter
"Can be Sold".

Fixes #2718, closes #2864