[DOC] Tweaks for StringIO#lineno= #609

Job	Run time
ruby-versions / Generate Ruby versions	4s
ubuntu-24.04 2.7	51s
ubuntu-24.04 3.0	13s
ubuntu-24.04 3.1	20s
ubuntu-24.04 3.2	16s
ubuntu-24.04 head	15s
ubuntu-24.04 3.3	16s
ubuntu-24.04 3.4	15s
ubuntu-22.04 2.7	50s
ubuntu-24.04 jruby-head	1m 3s
ubuntu-24.04 debug	25s
ubuntu-24.04 truffleruby-head	32s
ubuntu-22.04 3.3	17s
ubuntu-22.04 3.2	21s
ubuntu-22.04 3.0	19s
ubuntu-22.04 3.1	12s
ubuntu-22.04 3.4	14s
ubuntu-22.04 truffleruby-head	31s
ubuntu-22.04 head	19s
ubuntu-22.04 debug	24s
ubuntu-22.04 jruby-head	1m 5s
	9m 2s

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