review: add comments explaining helper functions

crates/ruff_linter/src/message/text.rs

@@ -295,6 +295,12 @@ fn replace_whitespace_and_unprintable(source: &str, annotation_range: TextRange)
     let mut range = annotation_range;
     let mut line_width = LineWidthBuilder::new(IndentWidth::default());
 
+    // Updates the range given by the caller whenever a single byte (at
+    // `index` in `source`) is replaced with `len` bytes.
+    //
+    // When the index occurs before the start of the range, the range is
+    // offset by `len`. When the range occurs after or at the start but before
+    // the end, then the end of the range only is offset by `len`.
     let mut update_range = |index, len| {
         if index < usize::from(annotation_range.start()) {
             range += TextSize::new(len - 1);
@@ -303,7 +309,8 @@ fn replace_whitespace_and_unprintable(source: &str, annotation_range: TextRange)
         }
     };
 
-    // impl_show_nonprinting!(('\x07', "␇"), ('\x08', "␈"), ('\x1b', "␛"), ('\x7f', "␡"));
+    // If `c` is an unprintable character, then this returns a printable
+    // representation of it (using a fancier Unicode codepoint).
     let unprintable_replacement = |c: char| -> Option<char> {
         match c {
             '\x07' => Some('␇'),