Make it clear that log enable's output file is per channel not per category (#196178)
It is natural to try:
```
log enable -f file1 lldb step
log enable -f file2 lldb process
```
or whatever, and then be surprised that all the logs end up in
`file2`... The help on `log enable`'s `-f` option didn't specify what
you were setting the file for. This PR just changes the help text to be
clear about the current state of things.