av1encode help menu missing key information

[greenjustin]

The av1encode help menu seems to be missing the "-t", "-m", "-u", "-d", "--syncmode", and "--LDB" options. "-t" is especially important for CBR rate control, so the absence of this information makes for a confusing user experience. It might also be useful for the help menu to give a list of valid values for the "--rcmode" flag.

There also appears to be some missing newlines for the sample usage at the end of the help dialog.

[XinfengZhang]

@leyu-yao