Add date format to If-Modified-Since documentation (#1196)

*  Fix If-Modified-Since example in API.md 

Fixes the example curl request date format. It didn't conform to the IMF-fix standard and was not accepted by the parseDate function.

Signed-off-by: Koen Tange <monokles@protonmail.ch>

* Add If-Modified-Since format description to API.md

Adds a short description elaborating on the different date formats supported by the If-Modified-Since header.

Signed-off-by: Koen Tange <monokles@protonmail.ch>

* Mark old time formats as obsolete in API.md

Clarified in the text that only the IMF-fixformat date format should be used in conjuction with the If-Modified-Since header.

Signed-off-by: Koen Tange <monokles@protonmail.ch>

docs/API.md

@@ -45,6 +45,13 @@ In any case a user doesn't have access to a requested entity, a 403 error will b
 ### If-Modified-Since
 
 Some index endpoints support limiting the result set to entries that have been changed since the given time.
+The supported date formats are:
+
+* IMF-fixdate:                 `Sun, 03 Aug 2019 10:34:12 GMT`
+* (obsolete) RFC 850:          `Sunday, 03-Aug-19 10:34:12 GMT`
+* (obsolete) ANSI C asctime(): `Sun Aug  3 10:34:12 2019`
+
+It is highly recommended to only use the IMF-fixdate format.
 
 Example curl request:
 
@@ -52,7 +59,7 @@ Example curl request:
 curl -u admin:admin -X GET \
     'http://localhost:8000/index.php/apps/deck/api/v1.0/boards/2/stacks' \
     -H "OCS-APIRequest: true" \
-    -H "If-Modified-Since: Mon, 5 Nov 2018 09:28:00 GMT"
+    -H "If-Modified-Since: Mon, 05 Nov 2018 09:28:00 GMT"
 ```
 
 # Endpoints