gh-126165: Improve docs of function math.isclose (#126215)

Co-authored-by: Sergey B Kirpichev <[email protected]>
Co-authored-by: Carol Willing <[email protected]>
Co-authored-by: Terry Jan Reedy <[email protected]>

Doc/library/cmath.rst

@@ -221,19 +221,21 @@ Classification functions
    ``False`` otherwise.
 
    Whether or not two values are considered close is determined according to
-   given absolute and relative tolerances.
+   given absolute and relative tolerances.  If no errors occur, the result will
+   be: ``abs(a-b) <= max(rel_tol * max(abs(a), abs(b)), abs_tol)``.
 
    *rel_tol* is the relative tolerance -- it is the maximum allowed difference
    between *a* and *b*, relative to the larger absolute value of *a* or *b*.
    For example, to set a tolerance of 5%, pass ``rel_tol=0.05``.  The default
    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.
-
-   If no errors occur, the result will be:
-   ``abs(a-b) <= max(rel_tol * max(abs(a), abs(b)), abs_tol)``.
+   within about 9 decimal digits.  *rel_tol* must be nonnegative and less
+   than ``1.0``.
+
+   *abs_tol* is the absolute tolerance; it defaults to ``0.0`` and it must be
+   nonnegative.  When comparing ``x`` to ``0.0``, ``isclose(x, 0)`` is computed
+   as ``abs(x) <= rel_tol  * abs(x)``, which is ``False`` for any ``x`` and
+   rel_tol less than ``1.0``.  So add an appropriate positive abs_tol argument
+   to the call.
 
    The IEEE 754 special values of ``NaN``, ``inf``, and ``-inf`` will be
    handled according to IEEE rules.  Specifically, ``NaN`` is not considered

Doc/library/math.rst

@@ -244,19 +244,21 @@ Number-theoretic and representation functions
    ``False`` otherwise.
 
    Whether or not two values are considered close is determined according to
-   given absolute and relative tolerances.
+   given absolute and relative tolerances.  If no errors occur, the result will
+   be: ``abs(a-b) <= max(rel_tol * max(abs(a), abs(b)), abs_tol)``.
 
    *rel_tol* is the relative tolerance -- it is the maximum allowed difference
    between *a* and *b*, relative to the larger absolute value of *a* or *b*.
    For example, to set a tolerance of 5%, pass ``rel_tol=0.05``.  The default
    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.
-
-   If no errors occur, the result will be:
-   ``abs(a-b) <= max(rel_tol * max(abs(a), abs(b)), abs_tol)``.
+   within about 9 decimal digits.  *rel_tol* must be nonnegative and less
+   than ``1.0``.
+
+   *abs_tol* is the absolute tolerance; it defaults to ``0.0`` and it must be
+   nonnegative.  When comparing ``x`` to ``0.0``, ``isclose(x, 0)`` is computed
+   as ``abs(x) <= rel_tol  * abs(x)``, which is ``False`` for any ``x`` and
+   rel_tol less than ``1.0``.  So add an appropriate positive abs_tol argument
+   to the call.
 
    The IEEE 754 special values of ``NaN``, ``inf``, and ``-inf`` will be
    handled according to IEEE rules.  Specifically, ``NaN`` is not considered