Documentation: admin: early_param()s are also listed in kernel-parameters

Add info that "early_param()" kernel boot parameters are also listed
in kernel-parameters.txt.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Link: https://lore.kernel.org/r/20201226174433.7885-1-rdunlap@infradead.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>