bcachefs: Add missing parameter description to bch2_bucket_alloc_trans()

bcachefs: Add missing parameter description to bch2_bucket_alloc_trans()

The function bch2_bucket_alloc_trans() lacked a description for the
nowait parameter in its documentation comment block. This patch adds the
missing description to ensure all parameters are properly documented.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=12179
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>