ndn-js: mark 'rarely used'

types/ndn-js/face.d.ts

@@ -48,7 +48,7 @@ export class Face {
     unsetInterestFilter(id: number): void;
 }
 
-export type ForwardingFlags = any;
+export type ForwardingFlags = any; // no declaration because this type is rarely used
 
 export class InterestFilter {
     constructor(filter: InterestFilter);

types/ndn-js/key-chain.d.ts

@@ -33,6 +33,7 @@ export class KeyChain {
     signWithSha256(interest: Interest): void;
 }
 
+// no declaration because these types are rarely used
 export type CertificateV2 = any;
 export type KeyParams = any;
 export type Pib = any;