Add position_in_class: after_doc to keep class doc adjacent to class

[yamat47]

Summary

Closes #201.

Add a new after_doc value for position_in_class (and the other position_in_* options). When set, the schema annotation is inserted above the contiguous documentation comment block that sits directly above the class declaration, so the human-written doc stays adjacent to the class.

The default behavior (before / top) is unchanged.

Problem

With the existing before / top positions, a documentation comment placed directly above the class declaration gets pushed above the schema annotation — away from the class it documents:

# Source file:
# Doc about User
class User < ApplicationRecord
end

# After annotaterb today:
# Doc about User
# == Schema Information
# ...
class User < ApplicationRecord
end

Fix

Setting position_in_class: after_doc slots the annotation above the doc block, keeping the doc next to the class:

# With position_in_class: after_doc:
# == Schema Information
# ...
# Doc about User
class User < ApplicationRecord
end

A "documentation comment" is defined as the contiguous comment block immediately above the class declaration with no blank line between. Recognized magic comments (encoding, frozen_string_literal, shareable_constant_value, warn_indent, typed, rbs_inline, plus Emacs/Vim modelines) are excluded so the annotation still slots between magic comments and the class doc:

# Source file:
# frozen_string_literal: true
# Doc about User
class User < ApplicationRecord
end

# With position_in_class: after_doc:
# frozen_string_literal: true
# == Schema Information
# ...
# Doc about User
class User < ApplicationRecord
end

[OdenTakashi]

Thanks for adding this!
One thing that still feels a little confusing to me is the name after_doc.

[OdenTakashi]

With the existing options, the names describe where the annotation appears when reading the file from top to bottom: before places it before the class block, and after places it after the class block.

Following that same logic, after_doc sounds like the annotation would appear after the doc block — in other words, between the doc and the class. But the actual behavior is the opposite.

I think before_doc feels more natural because it reads as “place the annotation before the doc block,” which matches what actually happens:

# == Schema Information  ← before the doc
# Doc about User
class User < ApplicationRecord
end

[yamat47]

@OdenTakashi

Thanks!

I confused myself with the naming here. Since the annotation is placed before the documentation comment (consistent with how before/after describe top-to-bottom placement), before_doc makes much more sense. Fixed in e5bb069 — renamed across the option values, CLI help, README, and specs.