token_metadata: Add docs for is_member

Make it clear, is_member checks if a node is part of the token ring and
checks nothing else.

locator/token_metadata.hh

@@ -169,6 +169,8 @@ public:
 
     void remove_endpoint(inet_address endpoint);
 
+    // Checks if the node is part of the token ring. If yes, the node is one of
+    // the nodes that owns the tokens and inside the set _normal_token_owners.
     bool is_member(inet_address endpoint) const;
 
     bool is_leaving(inet_address endpoint) const;