Mark note to translators properly and elaborate its contents.

* src/progress.c (create_image): Likewise

The note was not even picked by xgettext to be included in the .pot,
so barely any translators even saw the note.
Also, this is one of the most cryptic messages to translate in wget,
unless the translator happens to guess where the string is used,
or looks at the source.

Copyright-paperwork-exempt: Yes

src/progress.c

@@ -1143,8 +1143,10 @@ create_image (struct bar_progress *bp, double dl_total_time, bool done)
       int nbytes;
       int ncols;
 
-      /* Note to translators: this should not take up more room than
-         available here (6 columns).  Abbreviate if necessary.  */
+      /* TRANSLATORS: The meaning is "elapsed time", and it is shown
+       * next to the progress bar once the download is done.
+       * This should not take up more room than
+       * available here (6 columns).  Abbreviate if necessary.  */
       strcpy (p, _("    in "));
       nbytes = strlen (p);
       ncols  = count_cols (p);