Fix typos and improve text_helper documentation regarding sanitization

[jameslovejoy]

Summary

• Fixes doc typos in actionpack/lib/action_dispatch/routing/mapper.rb
• Fixes an example given for the highlight helper with sanitize: false
• Improves the introductory paragraph about sanitization in text_helper.rb

Other Information

With sanitize: false, the helper leaves the input HTML alone, but the documentation was incorrectly showing it as sanitized for the highlight helper.

The introductory sentence about sanitization said "Most text helpers by default sanitize the given content..." However, only 2 of the 11 helpers actually sanitize their given inputs, so I clarified it by adding "that generate HTML output" since that was the original reasoning when sanitization was added in 84d387b.

I'm open to any suggestions for clarifying the sanitization description further.

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