fix docstring for cache fixture regarding `/` on keys

_pytest/cacheprovider.py

@@ -192,8 +192,8 @@ def cache(request):
     cache.get(key, default)
     cache.set(key, value)
 
-    Keys must be strings not containing a "/" separator. Add a unique identifier
-    (such as plugin/app name) to avoid clashes with other cache users.
+    Keys must be a ``/`` separated value, where the first part is usually the
+    name of your plugin or application to avoid clashes with other cache users.
 
     Values can be any object handled by the json stdlib module.
     """