Document pthread barriers #12468

Job	Run time
extraction	0s
gobview (ubuntu-22.04, ocaml-variants.4.14.2+options,ocaml-option-flambda, 14)	7m 27s
regression (ubuntu-22.04, ocaml-variants.4.14.2+options,ocaml-option-flambda)	8m 0s
regression (macos-13, ocaml-variants.4.14.2+options,ocaml-option-flambda)	22m 21s
	37m 48s

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