Fix documentation of use of undef in form_with() and all_forms_with()

[jidanni]

"May not" is ambiguous. Indeed it is used in a different way at "this may not be an appropriate default". So I am guessing you mean "must not".

[coveralls]

Pull Request Test Coverage Report for Build 297

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 93.223%

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Totals
Change from base Build 294:	0.0%
Covered Lines:	729
Relevant Lines:	782

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💛 - Coveralls

[oalders]

Thanks @skaji and @jidanni!