docs: add missing entries to response aliases in context.md

[hustLer2k]

Add ctx.has(), ctx.vary() and ctx.writable to the list of response aliases for context. https://github.com/koajs/koa/blob/master/lib/context.js#L199-L216

Checklist

• ✔ I have ensured my pull request is not behind the main or master branch of the original repository.
• ✔ I have rebased all commits where necessary so that reviewing this pull request can be done without having to merge it first.
• ✔ I have written a commit message that passes commitlint linting.
• ✔ I have ensured that my code changes pass linting tests.
• ✔ I have ensured that my code changes pass unit tests.
• ✔ I have described my pull request and the reasons for code changes along with context if necessary.

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (dbf4b8f) 98.66% compared to head (7ed2ef0) 98.66%.

:exclamation: Current head 7ed2ef0 differs from pull request most recent head ec5e95f. Consider uploading reports for the commit ec5e95f to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1769 +/- ## ======================================= Coverage 98.66% 98.66% ======================================= Files 5 5 Lines 523 523 Branches 146 146 ======================================= Hits 516 516 Misses 7 7 ```

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