expr: Add comment why character is expected to be a backslash

Author: frendsick

src/uu/expr/src/syntax_tree.rs

@@ -184,6 +184,7 @@ impl StringOp {
                         },
                         '$' => {
                             if let Some('\\') = pattern_chars.peek() {
+                                // The next character was checked to be a backslash
                                 let backslash = pattern_chars.next().unwrap_or_default();
                                 match pattern_chars.peek() {
                                     // End of a capturing group