[DOC] Doc for StringIO#putc #621

Job	Run time
ruby-versions / Generate Ruby versions	5s
ubuntu-24.04 3.2	22s
ubuntu-24.04 3.0	15s
ubuntu-24.04 3.4	18s
ubuntu-24.04 3.1	20s
ubuntu-24.04 3.3	16s
ubuntu-22.04 2.7	47s
ubuntu-22.04 3.0	18s
ubuntu-22.04 3.4	25s
ubuntu-24.04 debug	32s
ubuntu-22.04 3.1	19s
ubuntu-22.04 jruby-head	1m 16s
ubuntu-24.04 2.7	48s
ubuntu-24.04 truffleruby-head	34s
ubuntu-22.04 head	17s
ubuntu-24.04 head	16s
ubuntu-22.04 3.2	22s
ubuntu-22.04 3.3	30s
ubuntu-22.04 debug	26s
ubuntu-24.04 jruby-head	1m 17s
ubuntu-22.04 truffleruby-head	36s
	10m 19s

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