Reland [Profiler] Improve the docstring for export_memory_timeline (#110983)
Summary: Add more details about the export_memory_timeline API, as we've landed new representations of the memory timeline data.
Test Plan: CI, should be no functional change, as we only changed comments.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/110983
Approved by: https://github.com/DanilBaibak