docs: Document /fetch_api_key API endpoint.

static/swagger/zulip.yaml

@@ -81,6 +81,49 @@ paths:
           schema:
             $ref: '#/definitions/EventsResponse'
 
+  /fetch_api_key:
+    post:
+      description: |
+                   Gather a token bound to a user account, to identify and
+                   authenticate them when making operations in the API.
+
+                   This token must be used as the password in the rest of the
+                   endpoints that require Basic authentication.
+      operationId: zerver.views.auth.api_fetch_api_key
+      parameters:
+      - name: username
+        in: formData
+        description: The email address for the user that owns the API key.
+        required: true
+        type: string
+      - name: password
+        in: formData
+        description: The password for the account of the API key's owner.
+        required: true
+        type: string
+      responses:
+        '200':
+          description: Success.
+          schema:
+            allOf:
+            - $ref: '#/definitions/JsonSuccess'
+            - required:
+              - api_key
+              - email
+            - properties:
+                api_key:
+                  type: string
+                  example: c2hvdWxuJ3QgdSBiZSB3b3JraW5nPzsp
+                email:
+                  description: |
+                               The email address for the user that owns the API
+                               key.
+                  type: string
+                  example: iago@zulip.com
+                msg:
+                  enum:
+                    - ''
+
   /messages:
     get:
       description: |