Start documenting libc.

Bug: N/A
Test: N/A
Change-Id: I17345cb72a5ffc3af1688cf5874589cfb1e1fea0
diff --git a/libc/include/sys/unistd.h b/libc/include/sys/unistd.h
index 1e823fb..eb7694d 100644
--- a/libc/include/sys/unistd.h
+++ b/libc/include/sys/unistd.h
@@ -1 +1,10 @@
+#pragma once
+
+/**
+ * @file sys/unistd.h
+ * @brief Historical synonym for `<unistd.h>`.
+ *
+ * New code should use `<unistd.h>` directly.
+ */
+
 #include <unistd.h>