Finish <stdio_ext.h>.

Implement __freading and __fwriting, and clarify the documentation that was
the cause of these not being implemented for years.

Bug: http://b/17157253
Test: ran tests
Change-Id: I89542c8131b13889e2585417a024050ecf2abcb7
diff --git a/libc/libc.arm.map b/libc/libc.arm.map
index d79e36b..06856c8 100644
--- a/libc/libc.arm.map
+++ b/libc/libc.arm.map
@@ -1320,6 +1320,8 @@
 
 LIBC_P {
   global:
+    __freading; # future
+    __fwriting; # future
     getlogin_r; # future
 } LIBC_O;