Documentation: kgdboc: Document new kgdboc_earlycon parameter

Documentation: kgdboc: Document new kgdboc_earlycon parameter

The recent patch ("kgdboc: Add kgdboc_earlycon to support early kgdb
using boot consoles") adds a new kernel command line parameter.
Document it.

Note that the patch adding the feature does some comparing/contrasting
of "kgdboc_earlycon" vs. the existing "ekgdboc".  See that patch for
more details, but briefly "ekgdboc" can be used _instead_ of "kgdboc"
and just makes "kgdboc" do its normal initialization early (only works
if your tty driver is already ready).  The new "kgdboc_earlycon" works
in combination with "kgdboc" and is backed by boot consoles.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/20200507130644.v4.9.I7d5eb42c6180c831d47aef1af44d0b8be3fac559@changeid
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>