[DOC] Tweaks for StringIO#each_line #567

Job	Run time
ruby-versions / Generate Ruby versions	4s
ubuntu-24.04 3.0	13s
ubuntu-24.04 3.2	26s
ubuntu-24.04 debug	23s
ubuntu-24.04 3.3	16s
ubuntu-24.04 2.7	27s
ubuntu-24.04 head	16s
ubuntu-24.04 truffleruby-head	32s
ubuntu-24.04 3.1	13s
ubuntu-22.04 2.7	29s
ubuntu-24.04 3.4	22s
ubuntu-24.04 jruby-head	1m 9s
ubuntu-22.04 3.0	14s
ubuntu-22.04 3.1	19s
ubuntu-22.04 3.2	18s
ubuntu-22.04 debug	19s
ubuntu-22.04 3.3	11s
ubuntu-22.04 3.4	19s
ubuntu-22.04 head	14s
ubuntu-22.04 truffleruby-head	25s
ubuntu-22.04 jruby-head	1m 6s
	8m 15s

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