docs: update 1.12.6 relnotes (#4520)

docs/release-notes/1.12.6.md

@@ -11,3 +11,8 @@
 - Fix {func}`~scanpy.pp.highly_variable_genes` (`flavor='seurat_v3'`), {func}`~scanpy.pp.calculate_qc_metrics`, and {func}`~scanpy.tl.rank_genes_groups` loading entire row-chunked {class}`dask.array.Array`s into memory when checking for count data or counting nonzero cells per gene {smaller}`Y Su` ({pr}`4446`)
 - Fix {func}`~scanpy.pp.calculate_qc_metrics` for sparse matrices of small integer dtypes, which gave negative `pct_counts_in_top_N_genes` (e.g. `int16`) or raised (e.g. `uint16`) {smaller}`P Paladugu` ({pr}`4461`)
 - Fix {func}`~scanpy.pp.neighbors` and {func}`~scanpy.tl.ingest` with `umap-learn` 0.6 {smaller}`P Angerer` ({pr}`4480`)
+- Multi-column `groupby` in {func}`~scanpy.pl.dotplot`, {func}`~scanpy.pl.matrixplot`,
+  {func}`~scanpy.pl.stacked_violin`, {func}`~scanpy.pl.tracksplot`, and
+  {func}`~scanpy.pl.heatmap` now accepts integer-categorical and numeric
+  observation columns instead of raising `TypeError` {smaller}`A Barlow` ({pr}`4385`)
+- Fix {func}`~scanpy.pp.combat` with categorical `covariates`, which produced a singular design matrix and either raised or silently returned wrong values {smaller}`L Heumos` ({pr}`4496`)