loader manuals: Clarify console and boot_serial

console=comconsole is for the loader and boot_serial is for the kernel.

I think we should centralize documentation on the loader settings in one
manual, such a loader.8. The current proliferation is not obvious where
to find the documentation. To the user, these are all set in loader.conf
or at the loader prompt.

MFC after:		3 days
Differential Revision:	https://reviews.freebsd.org/D53695

stand/defaults/loader.conf.5

@@ -343,6 +343,9 @@ selects the video console which prevents any input and hides all output
 replacing it with
 .Dq spinning
 character (useful for embedded products and such).
+This setting is for the
+.Xr loader 8
+only and does not set the kernel output.
 .It Va screen.font
 Set font size for framebuffer mode.
 The default font size is selected based on screen resolution.

stand/man/loader_simp.8

@@ -358,8 +358,8 @@ in
 .Xr kenv 1 ,
 not for any particular value.
 .It Va boot_serial
-Force the use of a serial console even when an internal console
-is present.
+Force the use of a serial console for the kernel
+even when an internal console is present.
 .It Va boot_single
 Prevents the kernel from initiating a multi-user startup; instead,
 a single-user mode will be entered when the kernel has finished