Clarify usage of parallel relationships

[jelhan]

This PR tries to implement the agreement we came up in #1361.

I struggled with the wording to be honest to be honest. I found it difficult to distinguish between the different kinds of parallel relationships. For sake of clarity let me show them in code:

1. Different relationships

   {
     "data": {
       "type": "articles",
       "id": "1",
       "relationships": {
         "author": {
           "data": {
             "type": "users",
             "id": "12"
           }
         },
        "editors": {
          "data": [
            {
              "type": "people",
              "id": "12"
            }
          ]
        }
     }
   }

2. Intermediate resource

   {
     "data": {
       "type": "teams",
       "id": "1",
       "relationships": {
         "members": {
           "data": {
             "type": "teamMembers",
             "id": "12"
           }
         },
        "editors": {
          "data": [
            {
              "type": "teamMembers",
              "id": "13"
            }
          ]
        }
     },
     "included": [
       {
         "type": "teamMembers",
         "id": "12",
         "attributes": {
           "startDate": "2021-01-01",
           "endDate": "2021-12-31",
         },
         "relationships": {
           "user": {
             "data": {
               "type": "users",
               "id": "23"
             }
           }
         }
       },
       {
         "type": "teamMembers",
         "id": "13",
         "attributes": {
           "startDate": "2022-06-01",
           "endDate": "2022-12-31",
         },
         "relationships": {
           "user": {
             "data": {
               "type": "users",
               "id": "23"
             }
           }
         }
       }
     ]
   }

3. Multiple, direct reference on same relationship

   {
     "data": {
       "type": "teams",
       "id": "1",
       "relationships": {
         "members": {
           "data": [
             {
               "type": "users",
               "id": "23",
               "meta": {
                 "startDate": "2021-01-01",
                 "endDate": "2021-12-31",
               }
             },
            {
               "type": "users",
               "id": "23",
               "meta": {
                 "startDate": "2022-06-01",
                 "endDate": "2022-12-31",
               }
             }
           ]
         }
       }
     }
   }

We only aim to discourage using the third type of parallel relationships. We recommend to use the second type instead. I'm not sure how well this is expressed in the current wording.

Open questions:

1. Do we want to backport this recommendation to v1.1?
2. Do we need a better wording to distinguish between relationships of a resource (author, editors) and references to resource in such relationships?

Closes #1361