doc: add reference from serialize_with/deserialize_with attrs

borsh-derive/src/lib.rs

@@ -282,11 +282,15 @@ struct B<K, V> {
 }
 ```
 
+###### usage (comprehensive example)
+
+[borsh/examples/serde_json_value.rs](https://github.com/near/borsh-rs/blob/master/borsh/examples/serde_json_value.rs) is
+a more complex example of how the attribute may be used.
+
 ###### interaction with `#[borsh(skip)]`
 
 `#[borsh(serialize_with = ...)]` is not allowed to be used simultaneously with `#[borsh(skip)]`.
 
-
 */
 #[proc_macro_derive(BorshSerialize, attributes(borsh))]
 pub fn borsh_serialize(input: TokenStream) -> TokenStream {
@@ -622,6 +626,11 @@ struct B<K: Hash + Eq, V> {
 }
 ```
 
+###### usage (comprehensive example)
+
+[borsh/examples/serde_json_value.rs](https://github.com/near/borsh-rs/blob/master/borsh/examples/serde_json_value.rs) is
+a more complex example of how the attribute may be used.
+
 ###### interaction with `#[borsh(skip)]`
 
 `#[borsh(deserialize_with = ...)]` is not allowed to be used simultaneously with `#[borsh(skip)]`.

borsh/examples/serde_json_value.rs

@@ -103,6 +103,7 @@ mod serde_json_value {
             BorshDeserialize,
         };
 
+        /// this is copy-paste of https://github.com/near/borsh-rs/blob/master/borsh/src/de/hint.rs#L2-L5
         fn hint_cautious<T>(hint: u32) -> usize {
             let el_size = core::mem::size_of::<T>() as u32;
             core::cmp::max(core::cmp::min(hint, 4096 / el_size), 1) as usize