Improve warning message for function used as React child

[raimatt]

Summary

How did you test this change?

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[eps1lon]

What do you think about also using the placeholder here in case it's not clear what functionName is referring to?

[react-sizebot]

Comparing: b080063...3e09108

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.js	=	6.68 kB	6.68 kB	=	1.83 kB	1.83 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	=	530.04 kB	530.04 kB	=	93.63 kB	93.63 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	6.69 kB	6.69 kB	=	1.83 kB	1.83 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js	=	654.48 kB	654.48 kB	=	115.34 kB	115.34 kB
facebook-www/ReactDOM-prod.classic.js	=	674.42 kB	674.42 kB	=	118.68 kB	118.68 kB
facebook-www/ReactDOM-prod.modern.js	=	664.84 kB	664.84 kB	=	117.02 kB	117.02 kB

Significant size changes

Includes any change greater than 0.2%:

Expand to show

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
test_utils/ReactAllWarnings.js	+0.23%	66.57 kB	66.72 kB	+0.48%	16.72 kB	16.80 kB

Generated by 🚫 dangerJS against 3e09108

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