Add some JSON::Fragment documentation #785

Job	Run time
Ruby memcheck	1m 1s
ruby-versions / Generate Ruby versions	2s
ubuntu-latest 2.7	1m 7s
ubuntu-latest 3.0	40s
ubuntu-latest 3.1	38s
ubuntu-latest 3.2	35s
ubuntu-latest 3.3	39s
ubuntu-latest 3.4	40s
ubuntu-latest head	1m 34s
ubuntu-latest jruby	1m 7s
ubuntu-latest jruby-head	1m 4s
macos-14 2.7	57s
macos-14 3.0	19s
macos-14 3.1	25s
macos-14 3.2	18s
macos-14 3.3	20s
macos-14 3.4	22s
macos-14 head	19s
macos-14 jruby	36s
macos-14 jruby-head	54s
windows-latest 2.7	4m 51s
windows-latest 3.0	2m 36s
windows-latest 3.1	3m 35s
windows-latest 3.2	2m 37s
windows-latest 3.3	3m 20s
windows-latest 3.4	3m 15s
windows-latest head	1m 31s
ubuntu-24.04-arm 3.4	1m 19s
macos-13 3.4	45s
windows-latest mswin	1m 8s
ubuntu-latest jruby-9.4	1m 9s
macos-latest truffleruby-head	45s
ubuntu-latest truffleruby-head	2m 4s
	42m 32s

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