Clarify rb_str_conv_enc() specs #305

Job	Run time
RuboCop	30s
specs (ubuntu, 3.0.6)	1m 27s
specs (ubuntu, 3.1.4)	1m 32s
specs (ubuntu, 3.2.2)	1m 28s
specs (ubuntu, 3.3.0)	1m 22s
specs (macos, 3.0.6)	1m 20s
specs (macos, 3.1.4)	1m 19s
specs (macos, 3.2.2)	1m 24s
specs (macos, 3.3.0)	1m 9s
specs (windows, 3.0.6)	4m 11s
specs (windows, 3.1.4)	4m 28s
specs (windows, 3.2.2)	4m 47s
specs (windows, 3.3.0)	4m 46s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 21s
	31m 4s

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