add docstring for the main xspline class

src/xspline/xspl.py

@@ -1,20 +1,37 @@
 from typing import Optional
 
-from numpy.typing import NDArray
-
 from xspline.bspl import clear_bspl_cache, get_bspl_funs
 from xspline.poly import get_poly_fun
 from xspline.xfunction import BasisXFunction
+from xspline.typing import NDArray
 
 
 class XSpline(BasisXFunction):
+    """Main class for xspline functions.
+
+    Parameters
+    ----------
+    knots
+        Knots of the spline.
+    degree
+        Degree of the spline.
+    ldegree
+        Left extrapolation polynomial degree.
+    rdegree
+        Right extrapolation polynomial degree.
+    coefs
+        The coefficients for linear combining the spline basis.
+
+    """
 
-    def __init__(self,
-                 knots: tuple[float, ...],
-                 degree: int,
-                 ldegree: Optional[int] = None,
-                 rdegree: Optional[int] = None,
-                 coefs: Optional[NDArray] = None) -> None:
+    def __init__(
+        self,
+        knots: tuple[float, ...],
+        degree: int,
+        ldegree: Optional[int] = None,
+        rdegree: Optional[int] = None,
+        coefs: Optional[NDArray] = None,
+    ) -> None:
         # validate inputs
         knots, degree = tuple(sorted(map(float, knots))), int(degree)
         if len(set(knots)) < 2:
@@ -37,8 +54,26 @@ def __init__(self,
         self.ldegree, self.rdegree = ldegree, rdegree
         super().__init__(funs, coefs=coefs)
 
-    def get_design_mat(self, x: NDArray,
-                       order: int = 0, check_args: bool = True) -> NDArray:
+    def get_design_mat(
+        self, x: NDArray, order: int = 0, check_args: bool = True
+    ) -> NDArray:
+        """Create design matrix from spline basis functions.
+
+        Parameters
+        ----------
+        x
+            Data points.
+        order
+            Order of differentiation/integration.
+        check_args
+            If ``True``, it will automatically check and parse the arguments.
+
+        Returns
+        -------
+        describe
+            Design matrix from spline basis functions.
+
+        """
         design_mat = super().get_design_mat(x, order, check_args)
         clear_bspl_cache()
         return design_mat