Document `flow-control` keyword in onion service descriptors

C tor has:

src/feature/hs/hs_descriptor.c:#define str_flow_control "flow-control"
src/feature/hs/hs_descriptor.c:  T01(str_flow_control, R3_FLOW_CONTROL, GE(2), NO_OBJ),

But the string flow-control appears nowhere in rend-spec-v3.

Maybe it's time to merge the key parts of prop 324 into our specs?

cc @dgoulet @mikeperry

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