[DOC] Doc for StringIO.getc #544

Job	Run time
ruby-versions / Generate Ruby versions	5s
ubuntu-24.04 3.0	26s
ubuntu-24.04 2.7	39s
ubuntu-24.04 3.1	20s
ubuntu-24.04 head	27s
ubuntu-24.04 3.3	36s
ubuntu-24.04 jruby-head	1m 15s
ubuntu-24.04 3.4	25s
ubuntu-24.04 debug	31s
ubuntu-22.04 2.7	34s
ubuntu-22.04 3.2	26s
ubuntu-22.04 3.0	25s
ubuntu-24.04 truffleruby-head	40s
ubuntu-24.04 3.2	34s
ubuntu-22.04 3.1	34s
ubuntu-22.04 head	22s
ubuntu-22.04 3.4	28s
ubuntu-22.04 3.3	21s
ubuntu-22.04 debug	38s
ubuntu-22.04 jruby-head	1m 23s
ubuntu-22.04 truffleruby-head	47s
	11m 56s

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