[algorithms.parallel.exec] Rephrase to avoid incorrect use of "may not".

Convert rationale sentence to a note.

Author: zygoloid

source/algorithms.tex

@@ -419,14 +419,16 @@
 which overrides the usual guarantee from \ref{intro.execution}
 that function executions do not overlap with one another.
 \end{note}
-Since \tcode{execution::unsequenced_policy} allows
+The behavior of a program is undefined if
+it invokes a vectorization-unsafe standard library function
+from user code
+called from a \tcode{execution::unsequenced_policy} algorithm.
+\begin{note}
+Because \tcode{execution::unsequenced_policy} allows
 the execution of element access functions
 to be interleaved on a single thread of execution,
 blocking synchronization, including the use of mutexes, risks deadlock.
-Thus, the synchronization with \tcode{execution::unsequenced_policy}
-is restricted as follows:
-vectorization-unsafe standard library functions may not be invoked
-by user code called from \tcode{execution::unsequenced_policy} algorithms.
+\end{note}
 
 \pnum
 The invocations of element access functions in parallel algorithms invoked with
@@ -503,14 +505,16 @@
 which overrides the usual guarantee from \ref{intro.execution}
 that function executions do not overlap with one another.
 \end{note}
-Since \tcode{execution::parallel_unsequenced_policy} allows
+The behavior of a program is undefined if
+it invokes a vectorization-unsafe standard library function
+from user code
+called from a \tcode{execution::parallel_unsequenced_policy} algorithm.
+\begin{note}
+Because \tcode{execution::parallel_unsequenced_policy} allows
 the execution of element access functions
 to be interleaved on a single thread of execution,
 blocking synchronization, including the use of mutexes, risks deadlock.
-Thus, the synchronization with \tcode{execution::parallel_unsequenced_policy}
-is restricted as follows:
-vectorization-unsafe standard library functions may not be invoked by user code
-called from \tcode{execution::parallel_unsequenced_policy} algorithms.
+\end{note}
 
 \pnum
 \begin{note}