Update Synapse Tuning docs to reflect automatic memory use configuration

docs/maintenance-synapse.md

@@ -86,7 +86,7 @@ Cache autotune is controlled primarily via the following variables `matrix_synap
 
 The `matrix_synapse_caches_autotuning_max_cache_memory_usage: ""` variable controls the maximum amount of ram Synapse is allowed to use before synapse aggressively tries to get rid of cache until you either hit `matrix_synapse_caches_autotuning_min_cache_ttl: ""` being the age of your cache or you hit your memory use target set by `matrix_synapse_caches_autotuning_target_cache_memory_usage: ""` 
 
-The playbook defaults these settings to `2048MB` for max use and `1024MB` as target and `5m` as the minimum age of cache entries that are allowed to be evicted by autotune. 
+The playbook defaults these settings to `1/8 of system ram up to 2048mb` for max use and `1/16 of system ram up to 1024MB` as target and `30s` as the minimum age of cache entries that are allowed to be evicted by autotune. 
 
 Users who wish to lower Synapses RAM footprint should look at lowering cache factor together with autotune variables. If your cache factor is too low for a given auto tune setting your caches will not reach auto tune thresholds and therefore autotune wont be doing its job. Therefore its recommended to when running auto tune have your cache factor be too big by a resonable margin instead as to be able to have full use of autotune.