added explanatory documentation for the topics function in the functi…

…on builder (#731)

src/types/log.rs

@@ -137,6 +137,14 @@ impl FilterBuilder {
     }
 
     /// Topics
+    /// An event topic is a part of the event logging mechanism used by smart contracts
+    /// to record information on the blockchain.
+    /// These logs are stored in transaction receipts and can be efficiently filtered and searched.
+    /// Each log entry has -up to- four "topics," which are 32-byte values used to index the log.
+    ///    Topic 0: The hash of the event signature.
+    ///    Topic 1: The from address (indexed).
+    ///    Topic 2: The to address (indexed).
+    ///    Data: The value (not indexed).
     pub fn topics(
         mut self,
         topic1: Option<Vec<H256>>,