[DOC] Tweaks for StringIO#internal_encoding #552

Job	Run time
ruby-versions / Generate Ruby versions	4s
ubuntu-24.04 3.2	31s
ubuntu-24.04 3.1	21s
ubuntu-24.04 2.7	25s
ubuntu-24.04 3.0	20s
ubuntu-24.04 3.4	20s
ubuntu-24.04 debug	32s
ubuntu-24.04 head	20s
ubuntu-22.04 3.4	18s
ubuntu-24.04 jruby-head	1m 6s
ubuntu-24.04 3.3	19s
ubuntu-22.04 head	15s
ubuntu-22.04 2.7	31s
ubuntu-22.04 3.1	18s
ubuntu-22.04 3.3	19s
ubuntu-22.04 truffleruby-head	27s
ubuntu-24.04 truffleruby-head	25s
ubuntu-22.04 3.2	26s
ubuntu-22.04 jruby-head	1m 12s
ubuntu-22.04 3.0	19s
ubuntu-22.04 debug	22s
	9m 10s

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