docs: revise and expand ImmutableDict docstring.

[rfritz]

I had difficulty using the ImmutableDict type based on the documentation. After studying the code and reviewing advice on the Discourse site, I wrote this. This is the second version; I have made it match the style of the existing dictionary documentation and removed the tutorial content.

[giordano]

Thanks for the contribution, but can we please have a more descriptive title? The current one is rather cryptic.

[rfritz]

Ooops! That was pretty obscure. I've updated the commit message on dict.jl following the procedure (and wincing at the warnings) given in "Changing a commit message." Will that serve?

[giordano]

Ok, I changed the title of the PR based on commit message. As reference for next time, you can do it by clicking on the "Edit" button in the top right corner of the page.

[rfritz]

oh, duh. thanks for the correction and information.

[MasonProtter]

Just some minor comments / tweaks

[LilithHafner]

Thanks for continuing to work on this, @rfritz! FYI, in the future you can continue using the previous pr and don't have to open a new one, that way all the conversation stays in the same place. We can keep working here now, though :)