PR #564 finetuning of back-end "availability" terminology (#565)

Author: soxofaan

extensions/federation/README.md

@@ -108,8 +108,10 @@ schema:
 
 ## Resources supported only by a subset of back-ends
 
-Every discoverable resource that is defined as an object and allows to contain additional properties, can list the subset of back-ends that permanently support or host the exposed resource/functionality.
-Examples of where this could apply to (**not** comprehensive):
+Discoverable resources can explicitly list the subset of back-ends that support or host the exposed resource or functionality with the property `federation:backends`.
+
+Schema-wise, this only applies to resources that are defined as an object and allow to contain additional properties.
+For example (**not** comprehensive):
 
 - `GET /collections`
 - `GET /collections/{id}`
@@ -198,9 +200,9 @@ This also covers the case where the federation supports splitting a process into
 ## Temporarily or unintentionally unavailable resources
 
 Resources and back-ends can be temporarily or unintentionally unavailable.
-It is especially important to communicate to users missing resources when compiling lists of resources across multiple back-ends.
-Clients will assume that all lists of resources are the a combination of all back-ends listed under`federation` in `GET /`.
-Federated APIs can expose if any of the back-ends is temporarily not available and thus is not part of the response.
+It is especially important to communicate to users missing resources when compiling a **list of resources** across multiple back-ends.
+Clients will assume that all lists of resources are a combination of all back-ends listed under `federation` in `GET /`.
+Federated APIs can expose if any of the back-ends was not available when building the resource listing response with the property `federation:missing`.
 
 Examples of where this could apply to (**not** comprehensive):
 
@@ -223,7 +225,7 @@ schema:
   properties:
     'federation:missing':
       description: >-
-        Lists all back-ends that were temporarily not considered in the response (e.g. because they were not accessible).
+        Lists all back-ends that were unexpectedly unavailable during compilation of the response.
         If not given or empty, all back-ends supporting this endpoint were considered for creating the response.
         Back-ends that are listed as offline in the capabilities still need to be listed here.
       type: array