[DOC] Tweaks for StringIO#getc #606

Job	Run time
ruby-versions / Generate Ruby versions	4s
ubuntu-24.04 3.0	18s
ubuntu-24.04 3.1	18s
ubuntu-24.04 2.7	45s
ubuntu-24.04 3.2	18s
ubuntu-24.04 3.4	17s
ubuntu-24.04 3.3	14s
ubuntu-24.04 debug	26s
ubuntu-24.04 jruby-head	1m 6s
ubuntu-22.04 head	12s
ubuntu-22.04 3.2	15s
ubuntu-22.04 3.0	17s
ubuntu-24.04 truffleruby-head	26s
ubuntu-22.04 3.1	15s
ubuntu-22.04 debug	25s
ubuntu-22.04 2.7	52s
ubuntu-22.04 truffleruby-head	23s
ubuntu-22.04 3.3	22s
ubuntu-22.04 jruby-head	1m 10s
ubuntu-22.04 3.4	13s
ubuntu-24.04 head	12s
	8m 48s

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