docs: Clarify rules for quoting string literals

[andrew-farries]

Update the documentation to clarify how to specify default values for the add column and create table operations.

[exekias]

perhaps it's worth adding an example? I believe the most common use case is to add a column without a default but with a fixed backfill value (up migration)

[andrew-farries]

I've added an example of an up SQL that uses a string literal and referenced it from the docs.