tipc: Revise comment justifying release of configuration spinlock

Comment-only change to better explain why TIPC's configuration lock is
temporarily released while activating support for network interfaces,
and why the existing activation code doesn't require rework.

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>