[DOC] Doc for StringIO.getc #485

Job	Run time
ruby-versions / Generate Ruby versions	3s
windows-latest 3.4	1m 25s
windows-latest jruby-head	2m 17s
windows-latest 3.1	1m 27s
windows-latest 3.0	1m 41s
windows-latest mingw	1m 20s
windows-latest mswin	1m 34s
windows-latest 3.2	1m 43s
windows-latest 2.7	2m 8s
windows-latest 3.3	1m 35s
	15m 13s

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