Issue #26483: Clarify str.isdecimal() and isdigit()

Patch by Julien Palard.

Doc/library/stdtypes.rst

@@ -1639,18 +1639,20 @@ expression support in the :mod:`re` module).
 
    Return true if all characters in the string are decimal
    characters and there is at least one character, false
-   otherwise. Decimal characters are those from general category "Nd". This category
-   includes digit characters, and all characters
-   that can be used to form decimal-radix numbers, e.g. U+0660,
-   ARABIC-INDIC DIGIT ZERO.
+   otherwise. Decimal characters are those that can be used to form
+   numbers in base 10, e.g. U+0660, ARABIC-INDIC DIGIT
+   ZERO.  Formally a decimal character is a character in the Unicode
+   General Category "Nd".
 
 
 .. method:: str.isdigit()
 
    Return true if all characters in the string are digits and there is at least one
    character, false otherwise.  Digits include decimal characters and digits that need
-   special handling, such as the compatibility superscript digits.  Formally, a digit
-   is a character that has the property value Numeric_Type=Digit or Numeric_Type=Decimal.
+   special handling, such as the compatibility superscript digits.
+   This covers digits which cannot be used to form numbers in base 10,
+   like the Kharosthi numbers.  Formally, a digit is a character that has the
+   property value Numeric_Type=Digit or Numeric_Type=Decimal.
 
 
 .. method:: str.isidentifier()