Issue #25179: Preparatory cleanup of existing docs on string formatting

Issue #25179: Preparatory cleanup of existing docs on string formatting

* There was a link pointing to the section on the string.Formatter class (and
  multiple links in Python 3), when the section on the common format string
  syntax is probably more appropriate
* Fix references to various format() functions and methods
* Nested replacement fields may contain conversions and format specifiers,
  and this is tested in Python 3; see Issue #19729 for instance