docs: Adding missing docstrings (#756)

* Add missing docstrings

* Added conftest

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: DeaMariaLeon

conftest.py

@@ -0,0 +1,14 @@
+import pytest
+
+
+@pytest.fixture(scope="session", autouse=True)
+def add_doctest_modules(doctest_namespace):
+    import sparse
+
+    import numpy as np
+
+    if sparse._BackendType.Numba != sparse._BACKEND:
+        pytest.skip()
+
+    doctest_namespace["np"] = np
+    doctest_namespace["sparse"] = sparse