Add comment for unsafe block in percent_encoding

Author: JoshMcguigan

percent_encoding/lib.rs

@@ -252,6 +252,8 @@ impl<'a> Iterator for PercentEncode<'a> {
                 self.bytes = remaining;
                 Some(percent_encode_byte(first_byte))
             } else {
+                // The unsafe blocks here are appropriate because the bytes are
+                // confirmed as a subset of UTF-8 in should_percent_encode.
                 for (i, &byte) in remaining.iter().enumerate() {
                     if self.ascii_set.should_percent_encode(byte) {
                         // 1 for first_byte + i for previous iterations of this loop