update what_does_check_do.md to mention \r escaping

Windows newline escaping was added in b3sum v1.5.5 last year
(1170f02), and this commit updates the check
doc to reflect that. Note that Coreutils added \r escaping in v9.0 in 2021
(coreutils/coreutils@ed1c584),
a year after the original check doc was written.

Author: oconnor663

b3sum/what_does_check_do.md

@@ -74,9 +74,11 @@ Notice two things. First, `b3sum` puts a single `\` character at the front of
 the line. This indicates that the filepath contains escape sequences that
 `b3sum --check` will need to unescape. Then, `b3sum` replaces the newline
 character in the filepath with the two-character escape sequence `\n`.
-Similarly, if the filepath contained a backslash, `b3sum` would escape it as
-`\\` in the output. So far, all of this behavior is still identical to
-`md5sum`.
+Similarly, if the filepath contained carriage returns or backslashes, `b3sum`
+would escape those as `\r` and `\\` in the output. So far, all of this behavior
+is still identical to `md5sum`. (Note: Coreutils [introduced `\r`
+escaping](https://github.com/coreutils/coreutils/commit/ed1c58427d574fb4ff0cb8f915eb0d554000ceeb)
+in v9.0, September 2021.)
 
 ## Invalid Unicode