Add an example of using coalesce() inside a by() modulator.

[krlawrence]

Should add an example that shows that this is possible

gremlin> g.V(0,3).project('city').by(coalesce(values('city'),constant('not found')))
==>[city:not found]
==>[city:Austin]

[Pippin555]

I have another usage for coalesce that will be useful for 'upsert', in python:

   airport_code = 'HLO'
   airport_description = 'World'

    t = (g.V()
         .has('airport', 'code', airport_code)
         .fold()       # "None" is folded if not found
         .coalesce(
            unfold(),  # if "None" is unfolded the 'else' part of coalesce step is taken
            g.addV('airport')
            .property('code', airport_code)
         )
         .property('description', airport_description)
    # ...
    )

    assert next(t, None)

[krlawrence]

As part of the changes to address this issue, we should also discuss the changes to the way "non productive" by modulators now act as filters in Gremlin post 3.6.0.