[DOC] Doc for StringIO#pread #561

Job	Run time
ruby-versions / Generate Ruby versions	3s
windows-latest 2.7	2m 34s
windows-latest 3.3	1m 24s
windows-latest 3.0	1m 31s
windows-latest 3.2	1m 31s
windows-latest 3.4	2m 12s
windows-latest 3.1	1m 59s
windows-latest mingw	1m 21s
windows-latest mswin	1m 20s
windows-latest jruby-head	2m 24s
	16m 19s

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