docs: add a comment explaining how timestamp check works

Signed-off-by: Gustavo Inacio <[email protected]>

tap_core/src/tap_receipt/checks.rs

@@ -78,6 +78,7 @@ impl Check for TimestampCheck {
     }
 }
 
+/// Timestamp Check verifies if the receipt is **greater or equal** than the minimum timestamp provided.
 pub struct BatchTimestampCheck(pub u64);
 
 impl CheckBatch for BatchTimestampCheck {