gh-130664: specify how thousands separators are used with 0-padding

Author: skirpichev

Doc/library/string.rst

@@ -449,7 +449,9 @@ If not specified, then the field width will be determined by the content.
 When no explicit alignment is given, preceding the *width* field by a zero
 (``'0'``) character enables sign-aware zero-padding for numeric types,
 excluding :class:`complex`.  This is equivalent to a *fill* character of
-``'0'`` with an *alignment* type of ``'='``.
+``'0'`` with an *alignment* type of ``'='``.  If thousands separators
+are used (as specified either by ``'_'`` or ``','`` option), they placed
+also between leading zeros.
 
 .. versionchanged:: 3.10
    Preceding the *width* field by ``'0'`` no longer affects the default