[DOC] Tweaks for StringIO#external_encoding #542

Job	Run time
ruby-versions / Generate Ruby versions	6s
ubuntu-24.04 3.3	32s
ubuntu-24.04 3.2	24s
ubuntu-24.04 3.0	27s
ubuntu-24.04 debug	32s
ubuntu-24.04 jruby-head	1m 20s
ubuntu-24.04 head	26s
ubuntu-22.04 3.2	30s
ubuntu-22.04 3.3	26s
ubuntu-24.04 2.7	34s
ubuntu-24.04 3.1	28s
ubuntu-22.04 3.1	28s
ubuntu-22.04 3.4	36s
ubuntu-24.04 3.4	29s
ubuntu-24.04 truffleruby-head	40s
ubuntu-22.04 3.0	25s
ubuntu-22.04 debug	33s
ubuntu-22.04 head	24s
ubuntu-22.04 2.7	38s
ubuntu-22.04 truffleruby-head	46s
ubuntu-22.04 jruby-head	1m 16s
	12m 0s

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