Add documentation referencing "Please specify a conversion expression to alter the type of property ...: cast_expr>"

[vonkoff]

I'd recommend putting some more information about handling migrations. Specifically when the prompt Please specify a conversion expression to alter the type of property ...: cast_expr> . It took me a while to get what this meant and how to handle it. There are sparse amounts of times it's mentioned in GitHub or discord. I didn't really find anything in the EdgeDB documentation regarding this until I did a google search and found it mentioned in the changelog here https://www.edgedb.com/docs/changelog/1_0_b1

I could see a newbie, such as myself, not being able to comprehend what they need to do and start from scratch with a new project init instead of going through with a migration.

Also, it'd be a big plus to see an example snippet being explained in the "Easy EdgeDB Book".

[riolly]

Yes, please. I am the example.
I try to migrate property from an array of strings
property important_places -> array<str>;
to a multi-property string.
multi property important_places -> str;
and need to provide the cast expression.

I'm newbie and still confuse to do it correctly.

But anyway the docs are still really awesome!

[riolly]

And also fill_expr