Add some context on malloc_info and RSS to heapprofd doc.
Add some context on malloc_info and RSS to heapprofd doc.

Change-Id: Iaa5afd578c5feb8555c423aaab14701b7d69e743
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto