[DOC] Doc for StringIO.size #569

Job	Run time
ruby-versions / Generate Ruby versions	4s
ubuntu-24.04 3.0	12s
ubuntu-24.04 2.7	29s
ubuntu-24.04 3.2	17s
ubuntu-24.04 3.3	17s
ubuntu-24.04 head	18s
ubuntu-24.04 debug	21s
ubuntu-22.04 3.2	14s
ubuntu-22.04 3.0	19s
ubuntu-24.04 jruby-head	1m 10s
ubuntu-24.04 3.4	14s
ubuntu-22.04 3.1	14s
ubuntu-22.04 3.4	20s
ubuntu-22.04 3.3	14s
ubuntu-24.04 truffleruby-head	33s
ubuntu-22.04 2.7	30s
ubuntu-22.04 debug	21s
ubuntu-22.04 jruby-head	1m 8s
ubuntu-24.04 3.1	18s
ubuntu-22.04 head	18s
ubuntu-22.04 truffleruby-head	26s
ruby-versions / Generate Ruby versions	4s
ubuntu-24.04 3.1	18s
ubuntu-24.04 3.0	12s
ubuntu-24.04 3.3	17s
ubuntu-24.04 truffleruby-head	33s
ubuntu-24.04 debug	21s
ubuntu-24.04 3.2	17s
ubuntu-24.04 head	18s
ubuntu-22.04 3.2	14s
ubuntu-22.04 3.0	19s
ubuntu-24.04 2.7	45s
ubuntu-22.04 2.7	43s
ubuntu-24.04 3.4	14s
ubuntu-24.04 jruby-head	1m 10s
ubuntu-22.04 3.1	14s
ubuntu-22.04 3.4	20s
ubuntu-22.04 3.3	14s
ubuntu-22.04 head	18s
ubuntu-22.04 debug	21s
ubuntu-22.04 truffleruby-head	26s
ubuntu-22.04 jruby-head	1m 8s
	17m 3s

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