Add documentation to explain generate_ascii

Signed-off-by: Michael X. Grey <greyxmike@gmail.com>

Author: mxgrey

rosidl_runtime_rs/src/string.rs

@@ -522,6 +522,15 @@ mod tests {
         }
     }
 
+    /// If we use char::arbitrary(g) then we can get back characters which are
+    /// outside the ASCII range. Since the length calculated by [`String`] is
+    /// based on the utf-8 length of the string (not based on the number of bytes
+    /// in the string), putting non-ASCII characters into it can skew the length
+    /// calculation in strange ways.
+    ///
+    /// This function generates only ASCII characters by cutting their values
+    /// off at 127. This does not give an even distribution of ASCII characters,
+    /// but that fact is not important for this smoke test.
     fn generate_ascii(g: &mut Gen) -> char {
         let c = u8::min(u8::arbitrary(g), 127);
         c as char