[Profiler] Improve the docstring for export_memory_timeline (#110949)
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.
Differential Revision: D50123450
Pull Request resolved: https://github.com/pytorch/pytorch/pull/110949
Approved by: https://github.com/davidberard98
