x86/Kconfig: Document CONFIG_PCI_MMCONFIG

This configuration option had no help text, so add it.

CONFIG_EXPERT is enabled on some distribution kernels, so people using a
distribution kernel's configuration as a starting point will see this
option.

[ mingo: Standardized the new Kconfig text a bit. ]

Signed-off-by: Mateusz Jończyk <mat.jonczyk@o2.pl>
Signed-off-by: David Heideberg <david@ixit.cz>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20250321-x86_x2apic-v3-5-b0cbaa6fa338@ixit.cz