bpo-31907: [doc] clarify that str.format() does not support arbitrary…

bpo-31907: [doc] clarify that str.format() does not support arbitrary… · python/cpython@fb1d01b

Original file line number	Diff line number	Diff line change
`@@ -188,8 +188,8 @@ Format String Syntax`
`188`	`188`	The :meth:`str.format` method and the :class:`Formatter` class share the same
`189`	`189`	syntax for format strings (although in the case of :class:`Formatter`,
`190`	`190`	`subclasses can define their own format string syntax). The syntax is`
`191`		-related to that of :ref:`formatted string literals <f-strings>`, but
`192`		`-there are differences.`
	`191`	+related to that of :ref:`formatted string literals <f-strings>`, but it is
	`192`	`+less sophisticated and, in particular, does not support arbitrary expressions.`
`193`	`193`
`194`	`194`	`.. index::`
`195`	`195`	`single: {} (curly brackets); in string formatting`