document sharded read config options in user-guide/config.rst

Author: aldenks

docs/user-guide/config.rst

@@ -33,6 +33,10 @@ Configuration options include the following:
 - Async and threading options, e.g. ``async.concurrency`` and ``threading.max_workers``
 - Selections of implementations of codecs, codec pipelines and buffers
 - Enabling GPU support with ``zarr.config.enable_gpu()``. See :ref:`user-guide-gpu` for more.
+- Tuning reads from sharded zarrs. When reading less than a complete shard, reads to nearby chunks
+  within the same shard will be combined into a single request if they are less than
+  ``sharding.read.coalesce_max_gap_bytes`` apart and the combined request size is less than
+  ``sharding.read.coalesce_max_bytes``.
 
 For selecting custom implementations of codecs, pipelines, buffers and ndbuffers,
 first register the implementations in the registry and then select them in the config.