Improve docs for `DeviceCapabilities.checksum`.

src/phy/mod.rs

@@ -209,7 +209,10 @@ pub struct DeviceCapabilities {
     /// dynamically allocated.
     pub max_burst_size: Option<usize>,
 
-    /// The set of protocols for which checksum can be computed in hardware.
+    /// Checksum behavior.
+    ///
+    /// If the network device is capable of verifying or computing checksums for some protocols,
+    /// it can request that the stack not do so in software to improve performance.
     pub checksum: ChecksumCapabilities,
 
     /// Only present to prevent people from trying to initialize every field of DeviceLimits,