doc: document exec* = spawn+exit bug with non-Cygwin Windows platforms Problem reported by Eli Zaretskii in: http://lists.gnu.org/archive/html/bug-grep/2014-05/msg00118.html * doc/posix-functions/execl.texi (execl): * doc/posix-functions/execle.texi (execle): * doc/posix-functions/execlp.texi (execlp): * doc/posix-functions/execv.texi (execv): * doc/posix-functions/execve.texi (execve): * doc/posix-functions/execvp.texi (execvp): Mention spawn+exit problem on non-Cygwin Windows platforms.