perf/hw_breakpoint: Fix DocBook warnings in perf hw_breakpoint

Fix the following W=1 kernel build warning(s):

  kernel/events/hw_breakpoint.c:461: warning: Function parameter or member 'context' not described in 'register_user_hw_breakpoint'
  kernel/events/hw_breakpoint.c:560: warning: Function parameter or member 'context' not described in 'register_wide_hw_breakpoint'

Signed-off-by: Haocheng Xie <xiehaocheng.cn@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20210527031947.1801-4-xiehaocheng.cn@gmail.com