add "empty list as catch-all" behavior to API docs.

[MoralCode]

Documented the usecase where an empty list passed to the children property of a route acts as a catch-all.

Source: https://gitter.im/kriasoft/universal-router?at=616713298c019f0d0b32350e

It might be useful to test the code examples since i mostly just guessed at what the return values would be instead of actually testing them.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [X] Documentation fix

Checklist:

• [ ] My code follows the code style of this project.
• [X] My change requires a change to the documentation.
• [X ] I have updated the documentation accordingly.
• [ ] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

[codecov-commenter]

Codecov Report

Merging #208 (07a7baa) into main (0dd2661) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##              main      #208   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines          229       229           
  Branches        66        66           
=========================================
  Hits           229       229           

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 0dd2661...07a7baa. Read the comment docs.