Document that "exec LIST" and "system LIST" may fall back to the shell on Win32

As noted on p5p [1] and subsequently discussed [2].

The Win32 functions for handling exec() and system() attempt to
specially handle shell builtins by catching spawn failures and
re-attempting the spawn using the shell with the given argument LIST.

If "exec PROGRAM LIST" syntax (or the equivalent for system()) is used,
then only the specified PROGRAM will ever be run (although Perl will
still try the spawn twice on Win32 if PROGRAM doesn't exist or otherwise
can't be executed).

[1] http://www.nntp.perl.org/group/perl.perl5.porters/2014/04/msg214556.html
[2] http://www.nntp.perl.org/group/perl.perl5.porters/2014/04/msg214564.html