Request 'params' are not encoded into URL (not documented)

I recently wrote some unit tests for functions that make API calls to a service using `requests`. The API calls also use `params` in their requests. 

For mocking this API I use `requests_mock` and the schema `mock://` as it is done in [their docs](https://requests-mock.readthedocs.io/en/latest/matching.html).

For some reason, the unit tests failed due to `NoMockAddress` while the logs say that requests without params were made, but in the actual code, params are used (see minimal example below). I took me quite a while to figure out that apparently params are only passed for URLs starting with `http` ([source](https://github.com/psf/requests/blob/79f60274f7e461b8fd2f579e741f748438d7eadb/requests/models.py#L384)).

If this is expected behavior, it would be great to have this documented somewhere, e.g. in [this section of the docs](https://docs.python-requests.org/en/latest/user/quickstart/#passing-parameters-in-urls).

Related to https://github.com/psf/requests/issues/1879

## Expected Result

Passed params are encoded into the URL.

## Actual Result

Passed params are **not** encoded into the URL. And: I could not find any documentation regarding this.

## Reproduction Steps

```python
import requests
import requests_mock

BASE_URL = "mock://myservice.com"


def list_items(limit: int = None):
    if limit:
        return requests.get(BASE_URL + "/items", params={"limit": limit})
    return requests.get(BASE_URL + "/items")


def test_list_items():
    with requests_mock.Mocker() as m:
        m.get(BASE_URL + "/items", json=[1, 2, 3, 4, 5])
        res = list_items()
        assert len(res.json()) == 5

        history = m.request_history[0]
        assert history.method == "GET"
        assert history.query == f""


def test_list_items_limit():
    with requests_mock.Mocker() as m:
        m.get(BASE_URL + "/items?limit=3", json=[1, 2, 3])
        res = list_items(3)
        assert len(res.json()) == 3

        history = m.request_history[0]
        assert history.method == "GET"
        assert history.query == "limit=3"
```

Running those tests with `pytest` results in an error for the second test:
```
requests_mock.exceptions.NoMockAddress: No mock address: GET mock://myservice.com/items
```

P.S. When replacing `mock://` with `http://` it works, of course (but it took me a long time to figure this out).

## System Information

    $ python -m requests.help

```json
{
  "chardet": {
    "version": "4.0.0"
  },
  "charset_normalizer": {
    "version": "2.0.10"
  },
  "cryptography": {
    "version": "3.4.7"
  },
  "idna": {
    "version": "3.3"
  },
  "implementation": {
    "name": "CPython",
    "version": "3.8.8"
  },
  "platform": {
    "release": "5.13.0-35-generic",
    "system": "Linux"
  },
  "pyOpenSSL": {
    "openssl_version": "101010bf",
    "version": "21.0.0"
  },
  "requests": {
    "version": "2.27.1"
  },
  "system_ssl": {
    "version": "101010df"
  },
  "urllib3": {
    "version": "1.26.8"
  },
  "using_charset_normalizer": false,
  "using_pyopenssl": true
}

```

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All of those examples use a session to make it work. They don't work with the functional API which means that your bug is invalid. We don't support any scheme because this is an HTTP library, not arbitrary URI library. That is why requests mock shows sessions in every example on the page you linked

Thanks for the fast response and clarification.

P.S. I could finally a statement documenting the usage of `params` with schemas other that `http://` within the `requests_mock` docs:
> [requests](http://python-requests.org/) only prepares URLs for http schemes (start with http and only contains letters, numbers, and the + and - signs). In particular params won’t work with the mock:// scheme, but will with http+mock://. 

([source](https://requests-mock.readthedocs.io/en/latest/adapter.html#adapter))