[issue31907] doc: Clarify error message when attempting to call function via str.format()

[Eric V. Smith]

Eric V. Smith <eric at trueblade.com> added the comment:

> The syntax is related to that of formatted string literals, but it is less sophisticated and, in particular, does not support arbitrary expressions.

That seems fine to me.

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<https://bugs.python.org/issue31907>
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