[sequence.reqmts] Convert advice on container selection into a note.

Also separate out the advice for std::array from that for std::vector and emphasize the main message.

Author: tvaneerd

source/containers.tex

@@ -755,18 +755,24 @@
 might define).
 
 \pnum
+\begin{note}
 The sequence containers
 offer the programmer different complexity trade-offs and should be used
 accordingly.
-\tcode{vector} or \tcode{array}
+\tcode{vector}
 is the type of sequence container that should be used by default.
+\tcode{array}
+should be used when the container has a fixed size known during translation.
 \tcode{list} or \tcode{forward_list}
 should be used when there are frequent insertions and deletions from the
 middle of the sequence.
 \tcode{deque}
 is the data structure of choice
 when most insertions and deletions take place at the beginning or at the
 end of the sequence.
+When choosing a container, remember \tcode{vector} is best;
+leave a comment to explain if you choose from the rest!
+\end{note}
 
 \pnum
 In Tables~\ref{tab:containers.sequence.requirements}