man: Document ulimit -v

Document that `ulimit` can set the `RLIMIT_AS` limit (virtual memory) with
the `-v` flag.

Fixes: https://bugs.debian.org/78556
Reported-by: Vincent Lefevre <vincent@vinc17.net>
Signed-off-by: Gioele Barabucci <gioele@svario.it>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>