The best replacement for rdoc is yard.

The best alternatives to rdoc are rated as following:
★★★★★  yard
★★★  ri
★★  markdown
★  tomdoc

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Speaking about rdoc, one probably shouldn't forget about gems.

Other search terms related to rdoc and their ratings are:
★★★★★ gems
You have in mind a companion for rdoc and it is not yet here? Propose!



Google Trends

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rdoc
yard
ri