Fix :stopdoc: directive being undone for C-defined classes

[st0012]

Summary

• After parsing a C file, parse_file resets done_documenting = false on all classes in top_level.classes_or_modules
• Since done_documenting= also sets @document_self = !value, this inadvertently resets document_self to true for classes that had :stopdoc: applied during parsing
• This became an issue after add_to_classes_or_modules was added to the C parser's handle_class_module, which caused C-defined classes to appear in top_level.classes_or_modules for the first time
• The fix tracks :stopdoc: directives with a @stopped_doc flag so done_documenting= preserves the stopdoc state

Reproduction

Run rdoc -C against a C extension that uses :stopdoc: around class definitions, such as io-console:

$ rdoc -C
The following items are not documented:

  class IO::ConsoleMode
  end

  module IO::generic_readable
  end

These classes are intentionally wrapped in /* :stopdoc: */ / /* :startdoc: */ and should not appear in coverage.

[matzbot]

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