ALSA: emu10k1: add separate documentation for E-MU cards

They are sufficiently different from Sound Blasters.

Signed-off-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Link: https://lore.kernel.org/r/20230825222157.170978-3-oswald.buddenhagen@gmx.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>