bpo-38657: Clarify numeric padding behaviour in string formatting (GH-17036)

Make the definition of the width more explicit that it includes any
extra signs added by other options.

https://bugs.python.org/issue38657

Automerge-Triggered-By: @Mariatta
(cherry picked from commit 424e568)

Co-authored-by: Pete Wicken <[email protected]>

Author: miss-islington

Doc/library/string.rst

@@ -415,8 +415,9 @@ error.
 .. versionchanged:: 3.6
    Added the ``'_'`` option (see also :pep:`515`).
 
-*width* is a decimal integer defining the minimum field width.  If not
-specified, then the field width will be determined by the content.
+*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