nep-393: add bool return to sbt_revoke_by_owner (#510)

In the
[reference](https://github.com/near-ndc/i-am-human/blob/cfc92b1/contracts/registry/src/registry.rs#L362)
implementation we return `bool` in the `registry.sbt_revoke_by_owner` to
signal if the operation should continue or it's finished.

We forgot to push this update to the standard.

This update also makes the API consistent with `sbt_recover` and
`sbt_soul_transfer`

neps/nep-0393.md

@@ -404,8 +404,15 @@ trait SBTRegistry {
     /// Must also emit `Burn` event if the SBT tokens are burned (removed).
     fn sbt_revoke(&mut self, tokens: Vec<TokenId>, burn: bool);
 
-    /// Similar to `sbt_revoke`, but revokes all `owner`s tokens issued by the caller.
-    fn sbt_revoke_by_owner(&mut self, owner: AccountId, burn: bool);
+    /// Similar to `sbt_revoke`. Allows SBT issuer to revoke all tokens by holder either by
+    /// burning or updating their expire time. When an owner has many tokens from the issuer,
+    /// the issuer may need to call this function multiple times, until all tokens are revoked.
+    /// Retuns true if all the tokens were revoked, false otherwise.
+    /// If false is returned issuer must call the method until true is returned
+    /// Must be called by an SBT contract.
+    /// Must emit `Revoke` event.
+    /// Must also emit `Burn` event if the SBT tokens are burned (removed).
+    fn sbt_revoke_by_owner(&mut self, owner: AccountId, burn: bool) -> bool;
 
     /// Allows issuer to update token metadata reference and reference_hash.
     /// * `updates` is a list of triples: (token ID, reference, reference base64-encoded sha256 hash).