[DOC] Tweaks for StringIO#lineno #608

Job	Run time
ruby-versions / Generate Ruby versions	4s
ubuntu-24.04 3.1	18s
ubuntu-24.04 2.7	5m 54s
ubuntu-24.04 3.4	20s
ubuntu-24.04 truffleruby-head	24s
ubuntu-24.04 3.3	16s
ubuntu-24.04 head	15s
ubuntu-24.04 jruby-head	1m 5s
ubuntu-22.04 2.7	42s
ubuntu-24.04 3.0	16s
ubuntu-22.04 3.2	16s
ubuntu-24.04 3.2	17s
ubuntu-22.04 3.3	16s
ubuntu-22.04 3.0	14s
ubuntu-24.04 debug	34s
ubuntu-22.04 3.4	22s
ubuntu-22.04 debug	22s
ubuntu-22.04 3.1	16s
ubuntu-22.04 truffleruby-head	25s
ubuntu-22.04 jruby-head	1m 5s
ubuntu-22.04 head	15s
	13m 56s

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