documentation: Add RCU_NONIDLE() restrictions to requirements

idle-loop code.

Steven Rostedt supplied <tt>_rcuidle</tt> event tracing,

which is used quite heavily in the idle loop.

However, there are some restrictions on the code placed within

<tt>RCU_NONIDLE()</tt>:

<ol>

<li>	Blocking is prohibited.

	In practice, this is not a serious restriction given that idle

	tasks are prohibited from blocking to begin with.

<li>	Although nesting <tt>RCU_NONIDLE()</tt> is permited, they cannot

	nest indefinitely deeply.

	However, given that they can be nested on the order of a million

	deep, even on 32-bit systems, this should not be a serious

	restriction.

	This nesting limit would probably be reached long after the

	compiler OOMed or the stack overflowed.

<li>	Any code path that enters <tt>RCU_NONIDLE()</tt> must sequence

	out of that same <tt>RCU_NONIDLE()</tt>.

	For example, the following is grossly illegal:

	<blockquote>

	<pre>

 1     RCU_NONIDLE({

 2       do_something();

 3       goto bad_idea;  /* BUG!!! */

 4       do_something_else();});

 5   bad_idea:

	</pre>

	</blockquote>

	<p>

	It is just as illegal to transfer control into the middle of

	<tt>RCU_NONIDLE()</tt>'s argument.

	Yes, in theory, you could transfer in as long as you also

	transferred out, but in practice you could also expect to get sharply

	worded review comments.

</ol>

<p>

It is similarly socially unacceptable to interrupt an