[Docs] Document mangling for the init accessor thunk

Author: janbaig

docs/ABI/Mangling.rst

@@ -368,6 +368,7 @@ Entities
   entity-spec ::= entity 'fa'                // runtime discoverable attribute generator
   entity-spec ::= 'fi'                       // non-local variable initializer
   entity-spec ::= 'fP'                       // property wrapper backing initializer
+  entity-spec ::= 'fF'                       // property wrapped field init accessor
   entity-spec ::= 'fW'                       // property wrapper init from projected value
   entity-spec ::= 'fD'                       // deallocating destructor; untyped
   entity-spec ::= 'fZ'                       // isolated deallocating destructor; untyped

test/Demangle/Inputs/manglings.txt

@@ -377,6 +377,7 @@ $s18resilient_protocol24ResilientDerivedProtocolPxAA0c4BaseE0Tn --> associated c
 $s3red4testyAA3ResOyxSayq_GAEs5ErrorAAq_sAFHD1__HCg_GADyxq_GsAFR_r0_lF ---> red.test<A, B where B: Swift.Error>(red.Res<A, B>) -> red.Res<A, [B]>
 $s3red4testyAA7OurTypeOy4them05TheirD0Vy5AssocQzGAjE0F8ProtocolAAxAA0c7DerivedH0HD1_AA0c4BaseH0HI1_AieKHA2__HCg_GxmAaLRzlF ---> red.test<A where A: red.OurDerivedProtocol>(A.Type) -> red.OurType<them.TheirType<A.Assoc>>
 $s17property_wrappers10WithTuplesV9fractionsSd_S2dtvpfP ---> property wrapper backing initializer of property_wrappers.WithTuples.fractions : (Swift.Double, Swift.Double, Swift.Double)
+$s16property_wrapper8MyStructV1xSivpfF ---> property wrapped field init accessor of property_wrapper.MyStruct.x : Swift.Int
 $sSo17OS_dispatch_queueC4sync7executeyyyXE_tFTOTA ---> {T:$sSo17OS_dispatch_queueC4sync7executeyyyXE_tFTO} partial apply forwarder for @nonobjc __C.OS_dispatch_queue.sync(execute: () -> ()) -> ()
 $s4main1gyySiXCvp ---> main.g : @convention(c) (Swift.Int) -> ()
 $s4main1gyySiXBvp ---> main.g : @convention(block) (Swift.Int) -> ()