[DOC] Doc for StringIO.size (#171) #583

Job	Run time
ruby-versions / Generate Ruby versions	5s
ubuntu-24.04 2.7	46s
ubuntu-24.04 3.2	20s
ubuntu-24.04 3.4	15s
ubuntu-24.04 head	14s
ubuntu-24.04 3.3	17s
ubuntu-24.04 3.0	12s
ubuntu-24.04 3.1	14s
ubuntu-24.04 debug	22s
ubuntu-24.04 truffleruby-head	23s
ubuntu-22.04 3.0	17s
ubuntu-22.04 2.7	42s
ubuntu-24.04 jruby-head	1m 34s
ubuntu-22.04 3.1	15s
ubuntu-22.04 3.2	17s
ubuntu-22.04 3.3	25s
ubuntu-22.04 head	18s
ubuntu-22.04 debug	25s
ubuntu-22.04 truffleruby-head	34s
ubuntu-22.04 3.4	19s
ubuntu-22.04 jruby-head	1m 7s
	9m 21s

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