bpo-35838: document optionxform must be idempotent (GH-12656)

bpo-35838: document optionxform must be idempotent (GH-12656) · python/cpython@9a838c5

Original file line number	Diff line number	Diff line change
`@@ -723,6 +723,12 @@ be overridden by subclasses or by attribute assignment.`
`723`	`723`	`>>> list(custom['Section2'].keys())`
`724`	`724`	`['AnotherKey']`
`725`	`725`
	`726`	`+ .. note::`
	`727`	`+ The optionxform function transforms option names to a canonical form.`
	`728`	`+ This should be an idempotent function: if the name is already in`
	`729`	`+ canonical form, it should be returned unchanged.`
	`730`	`+`
	`731`	`+`
`726`	`732`	`.. attribute:: ConfigParser.SECTCRE`
`727`	`733`
`728`	`734`	`A compiled regular expression used to parse section headers. The default`