Add preadv2/pwritev2 wrappers.

They're in glibc, though not in musl.

Also add basic doc comments to the whole of <sys/uio.h>.

Bug: http://b/203002492
Test: treehugger
Change-Id: Ic607f7f349e5b7c9bf66c25b7bd68f827da530d6
diff --git a/libc/libc.map.txt b/libc/libc.map.txt
index 8ede380..12b8114 100644
--- a/libc/libc.map.txt
+++ b/libc/libc.map.txt
@@ -1565,6 +1565,14 @@
     process_madvise;
 } LIBC_R;
 
+LIBC_T { # introduced=Tiramisu
+  global:
+    preadv2;
+    preadv64v2;
+    pwritev2;
+    pwritev64v2;
+} LIBC_S;
+
 LIBC_PRIVATE {
   global:
     __accept4; # arm x86