sched/completions/Documentation: Clean up the document some more

sched/completions/Documentation: Clean up the document some more

Refresh the document:

 - Remove unnecessary liguistic complexity and improve the clarity of the text

 - Improve the explanations all around

 - Remove unnecessary and stale version info

 - Fix whitespace noise

 - Make pseudo-code match kernel style

 - Fix minor syntax errors in pseudo-code

 - Use consistent denotation

 - Mark multi-CPU sequences more explicitly

 - Unbreak line breaks

 - Use quotes to refer to 'struct completion'

 - Use 'IRQ context' and 'IRQs' consistently

 - Improve grammar

 - etc.

Cc: John Garry <john.garry@huawei.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Nicholas Mc Guire <hofrat@osadl.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: corbet@lwn.net
Cc: linux-doc@vger.kernel.org
Link: http://lkml.kernel.org/r/1539183392-239389-1-git-send-email-john.garry@huawei.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>