Document decoded route literals

[funwithcthulhu]

Summary

• Document that route path literals are matched against decoded request paths.
• Add an expect test showing a percent-encoded request target matches the decoded route literal, while Dream.target and logger output can still preserve the original encoded target.

Closes #414.

Validation

• git diff --check upstream/master...HEAD
• opam exec -- dune runtest test/expect/server (fails locally because this switch is missing ppx_expect.config_types, ppx_expect, and lwt_ppx)