Documentation,ALSA Core
Currently arguments of the 'index' option from 0 through SNDRV_CARDS-1
force a module to take the specified index. Index -1 makes the module
take the first available index. This patch extends this convention so
that a negative index value is interpreted as a bitmask of the
permitted indexes. Special cases:

    -1 0xffffffff 0 and up
    -2 0xfffffffe 1 and up
    -4 0xfffffffc 2 and up
    ...

The patch includes also corrections of ALSA-Configuration.txt document.
Signed-off-by: Thomas Hood <jdthood@yahoo.co.uk>
Signed-off-by: Takashi Iwai <tiwai@suse.de>