Update documentation.

Fix the incorrect mention of when the zygote flag is set on a size.

Add information about how to use am dumpheap -n to dump the native
heap to a file.

Test: Documentation only change.

Change-Id: I82e810ee7ffffcf7506fb6fd9bcfdfb0465a48be

libc/malloc_debug/README.md

@@ -409,3 +409,15 @@ like two arguments instead of one.
     # setprop libc.debug.malloc.options backtrace
     # export LIBC_DEBUG_MALLOC_ENABLE 1
     # ls
+
+Enable malloc debug and dump the native allocation with backtraces to
+a file. This only works for zygote based java processes.
+
+    adb shell stop
+    adb shell setprop libc.debug.malloc.options backtrace
+    adb shell start
+    adb shell am dumpheap -n <PID_TO_DUMP> /data/local/tmp/heap.txt
+
+It is possible to use the backtrace\_enable\_on\_signal option as well,
+but it must be enabled through the signal before the file will contain
+any data.

libc/malloc_debug/README_api.md

@@ -50,5 +50,5 @@ The total number of these structures returned in *info* is
 *overall\_size* divided by *info\_size*.
 
 Note, the size value in each allocation data structure will have bit 31 set
-if this allocation was created by the Zygote process. This helps to distinguish
-between native allocations created by the application.
+if this allocation was created in a process forked from the Zygote process.
+This helps to distinguish between native allocations created by the application.