alloca.3: Add entry about defining VLAs in same block as alloca() to BUGS

Refer to alloca() as a (builtin) function or macro, as it could be
defined as either depending on the compiler.

Paragraph about bug comes from Darwin's libc, and example added to
illustrate it.

Reviewed by: bnovkov
Approved by: bnovkov
MFC after: 3 days
Obtained from: https://github.com/apple-oss-distributions/libc (partially)
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D55370

(cherry picked from commit 4da237aee328f368cd85b659854c4556a39f15ef)