[DOC] Tweaks for StringIO#pos (#193) #616

Job	Run time
ruby-versions / Generate Ruby versions	4s
ubuntu-24.04 3.2	15s
ubuntu-24.04 2.7	44s
ubuntu-24.04 3.1	11s
ubuntu-24.04 3.0	14s
ubuntu-24.04 truffleruby-head	27s
ubuntu-22.04 3.1	19s
ubuntu-24.04 3.4	24s
ubuntu-24.04 head	19s
ubuntu-24.04 3.3	16s
ubuntu-24.04 debug	22s
ubuntu-22.04 truffleruby-head	25s
ubuntu-22.04 3.0	11s
ubuntu-22.04 3.3	14s
ubuntu-22.04 3.2	21s
ubuntu-24.04 jruby-head	1m 10s
ubuntu-22.04 debug	26s
ubuntu-22.04 jruby-head	1m 3s
ubuntu-22.04 3.4	18s
ubuntu-22.04 head	19s
ubuntu-22.04 2.7	41s
	8m 43s

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