[DOC] Tweaks for StringIO#getc (#189) #619

Job	Run time
ruby-versions / Generate Ruby versions	5s
ubuntu-24.04 3.0	17s
ubuntu-24.04 2.7	42s
ubuntu-24.04 3.2	21s
ubuntu-24.04 3.1	11s
ubuntu-22.04 3.1	13s
ubuntu-22.04 3.0	17s
ubuntu-24.04 debug	23s
ubuntu-22.04 2.7	42s
ubuntu-24.04 head	17s
ubuntu-22.04 debug	29s
ubuntu-22.04 3.2	22s
ubuntu-22.04 3.4	16s
ubuntu-22.04 jruby-head	1m 5s
ubuntu-24.04 3.3	18s
ubuntu-22.04 3.3	19s
ubuntu-22.04 truffleruby-head	26s
ubuntu-24.04 3.4	17s
ubuntu-22.04 head	14s
ubuntu-24.04 truffleruby-head	31s
ubuntu-24.04 jruby-head	1m 6s
	8m 51s

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