gallium/docs: fix up transfer description for 1d arrays, add cube map arrays

gallium/docs: fix up transfer description for 1d arrays, add cube map arrays

Transfers always use z/depth for layers no matter if it's a 1d or 2d array
texture, we don't follow OpenGL's crazyness there. Luckily this appears to
only be a doc bug, everyone doing the right thing already.
While here also document z/depth parameter for cube map arrays.

v2: fix typo spotted by Eric Anholt

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>