sponge's man page should include a warning on how not to use it

[David D Lowe]

Using bash redirection with sponge does not work as expected. For example, this will make test_file empty:

sed 's/^/prefix/' test_file | sponge > test_file

Instead, one should do this instead:

sed 's/^/prefix/' test_file | sponge test_file

I feel the man page should be updated to include this warning.