docs: say when a selection read decodes more than the selection

The READMEs said ds[...] reads only the selected elements, and the
facade's read_selection docs that only intersecting chunks are
decompressed. The bounding-box path runs only when the box covers at
most half the dataset; larger boxes (any strided slice across the
dataset), compact, virtual and unwritten datasets and chunked ones with
a non-default fill value decode the whole dataset. The READMEs, the
facade and format docs, the bindings' docstrings and known-issues now
say so, and how index lists are read.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

docs/known-issues.md

@@ -7,6 +7,23 @@ deleting it.
 
 ---
 
+## Selection reads that decode more than the selection
+
+**Status:** open (documented 2026-09-26). `Dataset::read_selection` (and so
+the Python `ds[...]`) materialises only the selection's bounding box when
+that box covers at most half the dataset (`partial_read`). It decodes the
+whole dataset and extracts the selection instead when:
+- the bounding box covers more than half the dataset — which a strided
+  selection across a chunked dataset (`ds[::100]`) always does, although
+  it may touch few chunks;
+- the dataset is compact or virtual, or has no storage;
+- it is chunked with a non-default fill value (the box path does not fill
+  unallocated chunks, so the fill-aware full read is used).
+Values are correct in every case; this is cost only. Selections other than
+`Selection::All` also bypass the file's chunk cache. The bounding-box
+heuristic's other cost is measured under "Concurrent and contiguous read
+performance" below.
+
 ## Concurrent and contiguous read performance (measured 2026-09-26)
 
 **Status:** open. Measured on tank with `concurrent_read` against h5py