doc: say that printf(1) is preferred over echo(1)

doc: say that printf(1) is preferred over echo(1)

* src/echo.c (usage): Say printf(1) is preferred
due to being more standard and robust.
* man/echo.x [SEE ALSO]: Reference printf(1).
* doc/coreutils.texi (echo invocation): Mention in the
summary that echo is not robust when outputting
any string, and that printf is preferred.
Also expand on the examples showing how to
output a single '-n' string.
Addresses https://bugs.gnu.org/51311