Clarify rb_str_conv_enc() specs #1775

Job	Run time
RuboCop	31s
specs (ubuntu, 3.0.6)	1m 31s
specs (ubuntu, 3.1.4)	1m 30s
specs (ubuntu, 3.2.2)	1m 29s
specs (ubuntu, 3.3.0)	1m 22s
specs (macos, 3.0.6)	1m 36s
specs (macos, 3.1.4)	1m 21s
specs (macos, 3.2.2)	1m 7s
specs (macos, 3.3.0)	1m 37s
specs (windows, 3.0.6)	3m 42s
specs (windows, 3.1.4)	4m 7s
specs (windows, 3.2.2)	4m 24s
specs (windows, 3.3.0)	3m 37s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 25s
	29m 19s

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