key vs endkey

I believe that if `key` is provided, then `endkey` should be ignored? The docs seem to be unclear on this. However, the behavior I get is the opposite.

```js
await couch.put('a', {})
await couch.put('b', {})
await couch.put('c', {})

const { rows } = await couch.allDocs({
key: 'a',
endkey: 'b'
})
console.log(rows)
```

```
test/index.js 2> [
test/index.js 2> {
test/index.js 2> id: 'a',
test/index.js 2> key: 'a',
test/index.js 2> value: { rev: '1-967a00dff5e02add41819138abb3284d' }
test/index.js 2> },
test/index.js 2> {
test/index.js 2> id: 'b',
test/index.js 2> key: 'b',
test/index.js 2> value: { rev: '1-967a00dff5e02add41819138abb3284d' }
test/index.js 2> }
test/index.js 2> ]
```

Looks like a bug?

couchdb:3.1.1

Start by reproducing the allDocs({ key, endkey }) example from test/index.js against CouchDB 3.1.1, then read the allDocs API documentation and the related implementation. Confirm whether key and endkey should be combined or whether key takes precedence, and update the behavior or documentation so the result and tests match the documented semantics.

Written by the indexing model from the issue text.