Relax ForwardIterator requirements in FieldSequence

doc/types/FieldSequence.qbk

@@ -15,7 +15,7 @@ In this table:
 
 * `X` denotes a type that meets the requirements of [*FieldSequence].
 
-* `a` is a value of type `X`.
+* `c` is a value of type `X const`.
 
 [table FieldSequence requirements
 [[operation][type][semantics, pre/post-conditions]]
@@ -23,25 +23,33 @@ In this table:
     [`X::value_type`]
     []
     [
-        A type that meets the requirements of @b Field.
+        A type that meets the requirements of [link beast.ref.Field [*Field]].
     ]
 ]
 [
     [`X::const_iterator`]
     []
     [
-        A type that meets the requirements of @b ForwardIterator.
+        An iterator type whose `reference` type meets the
+        requirements of [link beast.ref.Field [*Field]], and which
+        satisfies all the requirements of [*ForwardIterator],
+        except that:
+
+        [ordered_list
+            [there is no requirement that `operator->` is provided, and]
+            [there is no requirement that `reference` be a reference type.]
+        ]
     ]
 ]
 [
-    [`a.begin()`]
+    [`c.begin()`]
     [`X::const_iterator`]
     [
         Returns an iterator to the beginning of the field sequence.
     ]
 ]
 [
-    [`a.end()`]
+    [`c.end()`]
     [`X::const_iterator`]
     [
         Returns an iterator to the end of the field sequence.