[DOC] Doc for StringIO#getbyte #484

Job	Run time
ruby-versions / Generate Ruby versions	5s
windows-latest 2.7	2m 11s
windows-latest 3.2	1m 45s
windows-latest 3.3	1m 30s
windows-latest 3.1	1m 45s
windows-latest mingw	1m 37s
windows-latest 3.4	1m 32s
windows-latest jruby-head	2m 47s
windows-latest 3.0	1m 39s
windows-latest mswin	2m 32s
	17m 23s

Provide feedback