docs: align SCM selector and tagged-build guidance

Signed-off-by: Ralf W. Grosse-Kunstleve <rgrossekunst@nvidia.com>

Author: rwgk

CONTRIBUTING.md

@@ -121,14 +121,16 @@ correct.** Packages select tags for their own release line:
 
 | Package | Tag pattern |
 | --- | --- |
-| `cuda-bindings` from `cuda_bindings_12/` | `v12.9.*` (e.g. `v12.9.10`) |
-| `cuda-bindings` from `cuda_bindings_13/` | `v13.4.*` (e.g. `v13.4.2`) |
-| `cuda-python` | The matching bindings selector, chosen with `CUDA_PYTHON_BUILD_MAJOR=12` or `13` (default) |
+| `cuda-bindings` from `cuda_bindings_12/` | `v12.*` (e.g. `v12.9.10`) |
+| `cuda-bindings` from `cuda_bindings_13/` | `v13.*` (e.g. `v13.4.2`) |
+| `cuda-python` | The matching bindings selector, chosen explicitly with `CUDA_PYTHON_BUILD_MAJOR=12` or `13`, or inferred as described below |
 | `cuda-core` | `cuda-core-v*` (e.g. `cuda-core-v1.1.0`) |
 | `cuda-pathfinder` | `cuda-pathfinder-v*` (e.g. `cuda-pathfinder-v1.6.0`) |
 
-Git source builds of `cuda-python` default to CUDA 13. Set
-`CUDA_PYTHON_BUILD_MAJOR=12` to select CUDA 12, or `13` explicitly for CUDA 13.
+Git source builds of `cuda-python` honor `CUDA_PYTHON_BUILD_MAJOR=12` or `13`
+when set. Without it, an exact `v12.*` tag at `HEAD` selects CUDA 12 when no
+`v13.*` tag points to that commit. Untagged commits and commits tagged for
+both majors default to CUDA 13.
 The resolved version determines dependencies: CUDA 12 depends on matching
 bindings, while CUDA 13 also depends on Core and Pathfinder. A standalone
 sdist retains its version and dependency major without requiring this variable.

cuda_bindings_13/docs/source/install.rst

@@ -138,7 +138,7 @@ Requirements
 
 [^3]: The version is derived from git tags via ``setuptools-scm``, so the clone
 must include a reachable tag matching the release line configured in this
-package's ``pyproject.toml`` (currently ``v13.4.*``). A tag from the other CUDA
+package's ``pyproject.toml`` (currently ``v13.*``). A tag from the other CUDA
 major does not determine this package's version. Do not use ``--depth`` or
 ``--no-tags``: a clone without matching reachable tags can build with a
 bogus version such as ``0.1.dev1+g0d22cb444``. See `Cloning the repository