Elaborate more in the docstring

src/include-sh/coerce.sh

     #
     # Replace non-printable characters with $COERCE__REPCHAR
     #
+    # Keep only characters that have grapgical representation ([:graph:] POSIX
+    # class), newline, tab and space.  Replace rest with $COERCE__REPCHAR.
+    #
     perl -CS -Mutf8 -pe "s|[^[:graph:] \t\n]|$COERCE__REPCHAR|g"
 }