bitmap: Align documentation between bitmap_gather() and bitmap_scatter()

bitmap: Align documentation between bitmap_gather() and bitmap_scatter()

The bitmap_scatter() mistakenly refers to itself for detailed explanation
about the relationships of two. Instead of simply fixing this, align text
in both making a cross-reference.

Fixes: de5f84338970 ("lib/bitmap: Introduce bitmap_scatter() and bitmap_gather() helpers")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Yury Norov <yury.norov@gmail.com>