Add note about fgets() in 1990/tbr

About why it's useful. It's a problem to keep gets() but it might be a
problem to not keep it as well. Nevertheless with macOS the output of
the alternative code (which probably should be deleted but this can
happen another time) is confusing:

    $ ./tbr.alt
    $ warning: this program uses gets(), which is unsafe.
    []

The [] is the cursor on a blank line. The first line of output of the
program (which is not supposed to have a newline) instead has the
warning and then a newline appended to it.

1990/tbr/README.md

@@ -23,8 +23,9 @@ make all
 modern compilers; `exit(3)` returns void but the function was used in a binary
 expression so this wouldn't even compile. Cody also changed the code to use
 `fgets()` instead of `gets()` so one would not get a warning about the use of
-`gets()` at execution which was interspersed with the output. Thank you Cody for
-your assistance!
+`gets()` at linking time or execution, the latter case causing confusing output
+due to the warning being interspersed with the program's output. Thank you Cody
+for your assistance!
 
 For the original version with `gets()` see the alternate code section below.