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Add detail to `cache_autotuning` config option documentation (#12776)

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changelog.d/12776.doc Normal file
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Add additional info to documentation of config option `cache_autotuning`.

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docs/usage/configuration/config_documentation.md
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@ -1130,14 +1130,19 @@ Caching can be configured through the following sub-options:
* `cache_autotuning` and its sub-options `max_cache_memory_usage`, `target_cache_memory_usage`, and
`min_cache_ttl` work in conjunction with each other to maintain a balance between cache memory
usage and cache entry availability. You must be using [jemalloc](https://github.com/matrix-org/synapse#help-synapse-is-slow-and-eats-all-my-ramcpu)
to utilize this option, and all three of the options must be specified for this feature to work.
to utilize this option, and all three of the options must be specified for this feature to work. This option
defaults to off, enable it by providing values for the sub-options listed below. Please note that the feature will not work
and may cause unstable behavior (such as excessive emptying of caches or exceptions) if all of the values are not provided.
Please see the [Config Conventions](#config-conventions) for information on how to specify memory size and cache expiry
durations.
* `max_cache_memory_usage` sets a ceiling on how much memory the cache can use before caches begin to be continuously evicted.
They will continue to be evicted until the memory usage drops below the `target_memory_usage`, set in
the flag below, or until the `min_cache_ttl` is hit.
* `target_memory_usage` sets a rough target for the desired memory usage of the caches.
the setting below, or until the `min_cache_ttl` is hit. There is no default value for this option.
* `target_memory_usage` sets a rough target for the desired memory usage of the caches. There is no default value
for this option.
* `min_cache_ttl` sets a limit under which newer cache entries are not evicted and is only applied when
caches are actively being evicted/`max_cache_memory_usage` has been exceeded. This is to protect hot caches
from being emptied while Synapse is evicting due to memory.
from being emptied while Synapse is evicting due to memory. There is no default value for this option.
Example configuration:
```yaml