[DOC] Doc for StringIO#read #623

Job	Run time
ruby-versions / Generate Ruby versions	3s
ubuntu-24.04 3.3	18s
ubuntu-24.04 2.7	50s
ubuntu-24.04 3.1	22s
ubuntu-24.04 3.0	20s
ubuntu-24.04 3.2	19s
ubuntu-24.04 3.4	16s
ubuntu-24.04 debug	26s
ubuntu-22.04 3.1	19s
ubuntu-24.04 truffleruby-head	30s
ubuntu-24.04 jruby-head	1m 10s
ubuntu-22.04 3.3	17s
ubuntu-22.04 2.7	50s
ubuntu-22.04 3.4	16s
ubuntu-22.04 3.0	19s
ubuntu-24.04 head	17s
ubuntu-22.04 3.2	19s
ubuntu-22.04 head	21s
ubuntu-22.04 jruby-head	1m 17s
ubuntu-22.04 truffleruby-head	33s
ubuntu-22.04 debug	29s
	9m 51s

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