manual/setjmp.texi: Clarify setcontext and signal handlers text

Calling setcontext from a signal handler can be done safely so
it is sufficient to note that it is not recommended.

Also mention in setcontext documentation that the behaviour of
setcontext when restoring a context created by a call to a signal
handler is unspecified.

2014-04-17  Will Newton  <will.newton@linaro.org>

	* manual/setjmp.texi (System V contexts): Add note that
	calling setcontext on a context created by a call to a
	signal handler is undefined.  Update text to note that
	setcontext from a signal handler is possible but not
	recommended.