[DOCS] Add special attributes for indices allocation filtering

docs/reference/index-modules/allocation.asciidoc

@@ -43,8 +43,9 @@ to be allocated to a node. This is in contrast to `include` which will
 include a node if ANY rule matches.
 
 The `include`, `exclude` and `require` values can have generic simple
-matching wildcards, for example, `value1*`. A special attribute name
-called `_ip` can be used to match on node ip values.
+matching wildcards, for example, `value1*`. Additonally, special attribute 
+names called `_ip`, `_name`, `_id` and `_host` can be used to match by node 
+ip address, name, id or host name, respectively.
 
 Obviously a node can have several attributes associated with it, and
 both the attribute name and value are controlled in the setting. For