Add x86_64 unwinding problems to Known Issues.

Bug: https://github.com/google/perfetto/issues/112
Change-Id: Iecac40fce4dec86d0bd4d7b55df0d53a3463d7a0

docs/data-sources/native-heap-profiler.md

diff --git a/docs/data-sources/native-heap-profiler.md b/docs/data-sources/native-heap-profiler.md
index 05dd4bb..5b605ee 100644
--- a/docs/data-sources/native-heap-profiler.md
+++ b/docs/data-sources/native-heap-profiler.md
@@ -466,7 +466,8 @@
 * For startup profiles, some frame names might be missing. This will be
   resolved in Android 12.
 * 32-bit programs cannot be targeted on 64-bit devices.
-* x86 platforms are not supported. This includes the Android _Cuttlefish_
+* x86 / x86_64 platforms are not supported. This includes the Android
+_Cuttlefish_.
   emulator.
 * On ARM32, the bottom-most frame is always `ERROR 2`. This is harmless and
   the callstacks are still complete.