gh-61449: document present 0-padding option behavior for builtin numeric types

Author: skirpichev

Doc/library/string.rst

@@ -428,10 +428,12 @@ The *width* is a decimal integer defining the minimum total field width,
 including any prefixes, separators, and other formatting characters.
 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 ``'='``.
+When no explicit *fill* character and/or 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 ``'='``.  If *fill* character
+is specified, ``'0'`` option is ignored.  If alignment is explicitly specified,
+it's value take preference over provided by ``'0'`` option.
 
 .. versionchanged:: 3.10
    Preceding the *width* field by ``'0'`` no longer affects the default