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:
https://github.com/element-hq/synapse/blob/4ac783549c5bac7a490a715d359f330bb0b1a161/synapse/metrics/jemalloc.py#L198-L201