feat: underscores as parameter names in receivers (#345)

pages/book/contracts.mdx

@@ -305,6 +305,18 @@ contract HelloWorld {
 }
 ```
 
+Naming a parameter of the receiver function with an underscore `_{:tact}` makes its value considered unused and discarded. This is useful when you don't need to inspect the message received and you only want it to convey a specific opcode:
+
+```tact
+message(42) UniverseCalls {}
+
+contract Example {
+    receive(_: UniverseCalls) {
+        // Got a Message with opcode 42
+    }
+}
+```
+
 ### Internal functions
 
 These functions behave similarly to private methods in popular object-oriented languages — they're internal to contracts and can be called by prefixing them with a special [identifier `self{:tact}`](#field-access). That's why internal functions can sometimes be referred to as "contract methods".

pages/book/receive.mdx

@@ -37,3 +37,15 @@ contract MyContract {
     }
 }
 ```
+
+Naming a parameter of the receiver function with an underscore `_{:tact}` makes its value considered unused and discarded. This is useful when you don't need to inspect the message received and you only want it to convey a specific opcode:
+
+```tact
+message(42) UniverseCalls {}
+
+contract Example {
+    receive(_: UniverseCalls) {
+        // Got a Message with opcode 42
+    }
+}
+```