Update IPNI_MH_SAMPLING.md

Document the string representation of multihash values (Base58).

Author: bajtos

IPNI_MH_SAMPLING.md

@@ -177,7 +177,7 @@ Samples a set of multihashes ingested by an IPNI indexer for a given provider ID
       federation protocol.
 
 - **Response**:
-    - `200 OK`: A list of sampled multihash objects, each containing information based on the input parameters.
+    - `200 OK`: A list of sampled multihash objects, each containing information based on the input parameters. The multihash values are encoded using the Base58 string representation.
     - `404 Not Found`: If the provider ID or context ID does not exist.
     - `400 Bad Request`: If the request parameters are invalid.
 
@@ -289,4 +289,4 @@ benefit of reducing the blast radius of churn in mutlihash records advertised by
 
 ## Copyright
 
-Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).
+Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).