Update docs on ignored history navigation

[daun]

Description

• Reword section on history entries to explain why swup ignores unknown history entries
• Add note about how a too specific specific link selector will create ignored history browser entries by the browser
• Fix some anchor links to options

Checks

• [x] The PR is submitted to the master branch
• [x] The code was linted before pushing (npm run lint)
• [x] All tests are passing (npm run test)

Additional information

• Should better help explain the behaviour seen in #853

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