Chore: Add pathIsRegexp option to docs, revert new trailing option as unneeded

[harryby1149]

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@titanism @3imed-jaberi PR to add new options to docs. Apologies, on reading through this again I realized we don't need to add a new "trailing" option. That trailing option comes from path-to-regexp, but is really just renaming the old strict option. Instead of handling them separately, we should add the trailing option to the Layer options Object, preserving the interface for end users.

[titanism]

v13.0.1 released https://github.com/koajs/router/releases/tag/v13.0.1