Added a note in the documentation suggesting that users may set
`PYTHONMALLOC=malloc` when using `jemalloc`. This allows jemalloc to
track memory usage more accurately by bypassing Python's internal
small-object allocator (`pymalloc`), helping to ensure that
`cache_autotuning` functions as expected.
This doc change aims to provide more clarity for users configuring
jemalloc with Synapse.
Based on:
4ac783549c/synapse/metrics/jemalloc.py (L198-L201)
