Reapply "drop doc-hidden on Strkey::MAX_ENCODED_LEN"

This reverts commit d622003.

Author: leighmcculloch

src/strkey.rs

@@ -36,7 +36,6 @@ impl Strkey {
     // SignedPayload is the longest strkey type.
     const MAX_PAYLOAD_LEN: usize = ed25519::SignedPayload::MAX_PAYLOAD_LEN;
     const MAX_BINARY_LEN: usize = binary_len(Self::MAX_PAYLOAD_LEN);
-    #[doc(hidden)]
     pub const MAX_ENCODED_LEN: usize = encode_len(Self::MAX_BINARY_LEN);
     const _ASSERTS: () = {
         assert!(Self::MAX_PAYLOAD_LEN == 100);