f Add comment explaining feature byte computation

Author: jkczyz

lightning/src/ln/features.rs

@@ -52,6 +52,14 @@ mod sealed {
 
 			impl Context for $context {
 				const KNOWN_FEATURE_FLAGS: &'static [u8] = &[
+					// For each byte, use bitwise-OR to compute the applicable flags for known
+					// required features `r_i` and optional features `o_j` for all `i` and `j` such
+					// that the following slice is formed:
+					//
+					// [
+					//  `r_0` | `r_1` | ... | `o_0` | `o_1` | ...,
+					//  ...,
+					// ]
 					$(
 						0b00_00_00_00 $(|
 							<Self as $required_feature>::REQUIRED_MASK)*
@@ -61,6 +69,8 @@ mod sealed {
 				];
 
 				const KNOWN_FEATURE_MASK: &'static [u8] = &[
+					// Similar as above, but set both flags for each feature regardless of whether
+					// the feature is required or optional.
 					$(
 						0b00_00_00_00 $(|
 							<Self as $required_feature>::REQUIRED_MASK |