gh-126165: Improve docs of function math.isclose

Doc/library/cmath.rst

@@ -229,8 +229,9 @@ Classification functions
    tolerance is ``1e-09``, which assures that the two values are the same
    within about 9 decimal digits.  *rel_tol* must be greater than zero.
 
-   *abs_tol* is the minimum absolute tolerance -- useful for comparisons near
-   zero. *abs_tol* must be at least zero.
+   *abs_tol* is the absolute tolerance; it must be nonnegative.  Only ``0.0``
+   is close to ``0.0`` per default.  Pass an appropriate absolute tolerance to
+   compare with ``0.0``.
 
    If no errors occur, the result will be:
    ``abs(a-b) <= max(rel_tol * max(abs(a), abs(b)), abs_tol)``.

Doc/library/math.rst

@@ -166,8 +166,9 @@ Number-theoretic and representation functions
    tolerance is ``1e-09``, which assures that the two values are the same
    within about 9 decimal digits.  *rel_tol* must be greater than zero.
 
-   *abs_tol* is the minimum absolute tolerance -- useful for comparisons near
-   zero. *abs_tol* must be at least zero.
+   *abs_tol* is the absolute tolerance; it must be nonnegative.  Only ``0.0``
+   is close to ``0.0`` per default.  Pass an appropriate absolute tolerance to
+   compare with ``0.0``.
 
    If no errors occur, the result will be:
    ``abs(a-b) <= max(rel_tol * max(abs(a), abs(b)), abs_tol)``.