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]>

crates/clawhdf5-py/src/select.rs

@@ -1,5 +1,7 @@
 //! h5py-style indexing (`ds[1, 2:10:3, ...]`) mapped onto hyperslab
-//! selections, so only the selected elements are read.
+//! selections, so the library reads the selection rather than the whole
+//! dataset (it still decodes everything for large selections; see the
+//! facade's `Dataset::read_selection`).
 //!
 //! The rules and error messages follow h5py's `selections.py`: integers
 //! (negative from the end) drop their axis, slices must have a positive