Rename threepidCreds to threepid_creds and get rid of array (#3471)

This fixes the behaviour to match what synapse implements in practice.
If you use threepidCreds, you will just get an error about a missing
threepid_creds field. Synapse also treats this as an object. All clients
also seem to send threepid_creds, if they work on Synapse. Since
matrix.org requires an email currently for registration, most clients
that implement registration, will hit this issue.

a0f48ee89d/synapse/handlers/ui_auth/checkers.py (L145)

fixes #3156
fixes #2189

Signed-off-by: Nicolas Werner <nicolas.werner@hotmail.de>

changelogs/client_server/newsfragments/3471.clarification

@@ -0,0 +1 @@
+Fix documentation errors around `threepid_creds`.

content/client-server-api/_index.md

@@ -719,14 +719,12 @@ follows:
 ```json
 {
   "type": "m.login.email.identity",
-  "threepidCreds": [
-    {
-      "sid": "<identity server session id>",
-      "client_secret": "<identity server client secret>",
-      "id_server": "<url of identity server authed with, e.g. 'matrix.org:8090'>",
-      "id_access_token": "<access token previously registered with the identity server>"
-    }
-  ],
+  "threepid_creds": {
+    "sid": "<identity server session id>",
+    "client_secret": "<identity server client secret>",
+    "id_server": "<url of identity server authed with, e.g. 'matrix.org:8090'>",
+    "id_access_token": "<access token previously registered with the identity server>"
+  },
   "session": "<session ID>"
 }
 ```
@@ -750,14 +748,12 @@ follows:
 ```json
 {
   "type": "m.login.msisdn",
-  "threepidCreds": [
-    {
-      "sid": "<identity server session id>",
-      "client_secret": "<identity server client secret>",
-      "id_server": "<url of identity server authed with, e.g. 'matrix.org:8090'>",
-      "id_access_token": "<access token previously registered with the identity server>"
-    }
-  ],
+  "threepid_creds": {
+    "sid": "<identity server session id>",
+    "client_secret": "<identity server client secret>",
+    "id_server": "<url of identity server authed with, e.g. 'matrix.org:8090'>",
+    "id_access_token": "<access token previously registered with the identity server>"
+  },
   "session": "<session ID>"
 }
 ```