fix: clearer description of array.target_shard_size (#3599)

Co-authored-by: Davis Bennett <davis.v.bennett@gmail.com>

Author: ilan-gold

docs/user-guide/performance.md

@@ -81,7 +81,7 @@ z6 = zarr.create_array(store={}, shape=(10000, 10000, 1000), shards=(1000, 1000,
 print(z6.info)
 ```
 
-`shards` can be `"auto"` as well, in which case the `array.target_shard_size_bytes` setting can be used to control the size of shards (i.e., the size of the shard will be as close to without being bigger than `target_shard_size_bytes`); otherwise, a default is used.
+`shards` can be `"auto"` as well, in which case the `array.target_shard_size_bytes` setting can be used to control the size of shards (i.e., the size of the chunks cumulatively and uncompressed within the shard will be as close to, without being bigger than, `array.target_shard_size_bytes`); otherwise, a default is used.
 
 ### Chunk memory layout