Docs: _ttl ignores all parameters except for enabled and default.

Also, add a line about that it can never be disabled after it was enabled.

docs/reference/mapping/fields/ttl-field.asciidoc

@@ -5,6 +5,8 @@ A lot of documents naturally come with an expiration date. Documents can
 therefore have a `_ttl` (time to live), which will cause the expired
 documents to be deleted automatically.
 
+`_ttl` accepts two parameters which are described below, every other setting will be silently ignored.
+
 [float]
 ==== enabled
 
@@ -20,12 +22,7 @@ should be defined:
 }
 --------------------------------------------------
 
-[float]
-==== store / index
-
-By default the `_ttl` field has `store` set to `true` and `index` set to
-`not_analyzed`. Note that `index` property has to be set to
-`not_analyzed` in order for the purge process to work.
+`_ttl` can only be enabled once and never be disabled again.
 
 [float]
 ==== default