docs: clarify nullable override behavior

[andrewmbenton]

• also remove some dead code

[daison12006013]

@andrewmbenton is there a documentation as well if we can re-use a certain override block in sqlc.yaml?

overrides:
  go:
    overrides:
      - column: "profiles.payment_flag"
      - column: "profileviews.is_active_member"
      - column: "profileviews.is_active_profile"
        nullable: true
        go_type:
          import: "gopkg.in/guregu/null.v4"
          package: "null"
          type: "Bool"
      
      - db_type: "varchar"
      - db_type: "text"
      - db_type: "json"
        nullable: true
        go_type:
          import: "gopkg.in/guregu/null.v4"
          package: "null"
          type: "String"

As an example above, where in the 3 columns will have a null.Bool.

Meanwhile I am using anchors, somewhat like this

      # Boolean specific columns
      - column: "profiles.payment_flag"
        nullable: true
        go_type: &nullBoolean
          import: "gopkg.in/guregu/null.v4"
          package: "null"
          type: "Bool"
      - column: "profileviews.is_active_member"
        nullable: true
        go_type: *nullBoolean
      - column: "profileviews.is_active_profile"
        nullable: true
        go_type: *nullBoolean

      # String
      - db_type: "varchar"
        nullable: true
        go_type: &nullString
          import: "gopkg.in/guregu/null.v4"
          package: "null"
          type: "String"
      - db_type: "text"
        nullable: true
        go_type: *nullString
      - db_type: "json"
        nullable: true
        go_type: *nullString

[andrewmbenton]

As an example above, where in the 3 columns will have a null.Bool.

This "fallthrough" behavior won't work as far as I know, although I think it is technically possible to implement. The anchors solution seems like the best way to reduce redundancy at the moment.