ENH: consistent handling of "value" for faePatchField

- base level now explicity equivalent to LAZY_READ with overriding as
  required

- clearer documentation for reading of "value" for faPatchField

src/finiteArea/fields/faPatchFields/constraint/processor/processorFaPatchField.C

@@ -101,7 +101,7 @@ Foam::processorFaPatchField<Type>::processorFaPatchField
     const dictionary& dict
 )
 :
-    coupledFaPatchField<Type>(p, iF, dict),
+    coupledFaPatchField<Type>(p, iF, dict, IOobjectOption::NO_READ),
     procPatch_(refCast<const processorFaPatch>(p, dict)),
     sendRequest_(-1),
     recvRequest_(-1)
@@ -116,6 +116,12 @@ Foam::processorFaPatchField<Type>::processorFaPatchField
             << " in file " << this->internalField().objectPath()
             << exit(FatalIOError);
     }
+
+    // Use 'value' supplied, or set to internal field
+    if (!this->readValueEntry(dict))
+    {
+        faPatchField<Type>::patchInternalField(*this);
+    }
 }