[DOC] Tweaks for StringIO.getbyte #605

Job	Run time
ruby-versions / Generate Ruby versions	4s
ubuntu-24.04 3.0	17s
ubuntu-24.04 2.7	47s
ubuntu-24.04 3.2	21s
ubuntu-24.04 jruby-head	1m 5s
ubuntu-24.04 head	13s
ubuntu-24.04 3.1	14s
ubuntu-24.04 3.3	16s
ubuntu-22.04 2.7	45s
ubuntu-24.04 truffleruby-head	27s
ubuntu-22.04 debug	22s
ubuntu-22.04 3.2	19s
ubuntu-24.04 debug	26s
ubuntu-22.04 3.0	14s
ubuntu-22.04 3.3	20s
ubuntu-24.04 3.4	20s
ubuntu-22.04 head	15s
ubuntu-22.04 3.1	13s
ubuntu-22.04 truffleruby-head	30s
ubuntu-22.04 jruby-head	1m 4s
ubuntu-22.04 3.4	18s
	8m 50s

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