[PATCH] manual: Update alloca and variable length array documentation

[Paul Eggert]

Thanks, this looks good, with one quibble:
> +Compared to @code{malloc}, variable length arrays share the same
> +advantages and disadvantages as @code{alloca}.  In particular, there is
> +no error checking (and security vulnerabilities can result from large
> +allocation requests), and some @nongnusystems{} do not support variable
> +length arrays because they only support earlier versions of ISO C which
> +do not include variable length arrays.

The last 2 lines are obsolete. Although C99 required VLAs, a standard 
C11 implementation that defines __STDC_NO_VLA__ need not support VLAs. I 
suggest removing " because they only support earlier versions of ISO C 
which do not include variable length arrays".