Document another obsolete POSIX function.

ualarm(3) was removed from POSIX in 2008, but somehow wasn't included in
the list of such functions (perhaps because it's not inherently useless
like a lot of the other stuff is; it just has a better replacement).

Test: N/A
Change-Id: I0347d3e7f1357bc2acb870f74e9084872c28ca3e
diff --git a/docs/status.md b/docs/status.md
index bf246a6..de1eed1 100644
--- a/docs/status.md
+++ b/docs/status.md
@@ -29,6 +29,7 @@
   * `gethostid`
   * `shm_open`/`shm_unlink`
   * `sockatmark`
+  * `ualarm`
 
 Missing functionality:
   * `<aio.h>`