update with -Wdocumentation.

Some clients use this, and it was breaking them if they tried to use
some of our headers (specifically 1.0 service manager). Putting this
up to unblock those changes.

Bug: N/A
Test: mma (-Wdocumentation checked at build time)
Change-Id: I4453848bf91f1d20d0c5c1a28dd9f9e6f5de99fc
diff --git a/Android.bp b/Android.bp
index 0f91348..fd750fa 100644
--- a/Android.bp
+++ b/Android.bp
@@ -16,6 +16,7 @@
     name: "libhidl-defaults",
     cflags: [
         "-Wall",
+        "-Wdocumentation", // since some users use this
         "-Werror",
         "-Wextra-semi",
     ],