Python: Add note about / for Django upload_to

I did a test locally, something like

    import requests

    req = requests.Request(
        "POST",
        "http://127.0.0.1:8000/app/upload-test/",
        data={"name": "foo"},
        files={"upload" : ("wat/haha|!#$%^&", open("foo.txt", "rb"))},
    )

    # print(req.prepare().body.decode('ascii'))

    requests.session().send(req.prepare())

and the `wat/` part was stripped from the filename

python/ql/lib/semmle/python/frameworks/Django.qll

@@ -2272,6 +2272,12 @@ module PrivateDjango {
    * A parameter that accepts the filename used to upload a file. This is the second
    * parameter in functions used for the `upload_to` argument to a `FileField`.
    *
+   * Note that the value this parameter accepts cannot contain a slash. Even when
+   * forcing the filename to contain a slash when sending the request, django does
+   * something like `input_filename.split("/")[-1]` (so other special characters still
+   * allowed). This also means that although the return value from `upload_to` is used
+   * to construct a path, path injection is not possible.
+   *
    * See
    *  - https://docs.djangoproject.com/en/3.1/ref/models/fields/#django.db.models.FileField.upload_to
    *  - https://docs.djangoproject.com/en/3.1/topics/http/file-uploads/#handling-uploaded-files-with-a-model