Clarify iterative functions (#1224)

Author: JPryce-Aklundh

modules/ROOT/pages/functions/list.adoc

@@ -230,16 +230,23 @@ Three lists of numbers in the given ranges are returned.
 
 .Details
 |===
-| *Syntax* 3+| `reduce(accumulator, variable)`
+| *Syntax* 3+| `reduce(accumulator = initial, variable IN list \| expression)`
 | *Description* 3+| Runs an expression against individual elements of a `LIST<ANY>`, storing the result of the expression in an accumulator.
-.3+| *Arguments* | *Name* | *Type* | *Description*
-| `accumulator` | `ANY` | A variable that holds the result as the list is iterated.
-| `variable` | `LIST<ANY>` | A variable that can be used within the reducing expression.
+.6+| *Arguments* | *Name* | *Type* | *Description*
+| `accumulator` | `ANY` | A variable that holds the result as the `list` is iterated.
+Starts with an `initial` value.
+| `initial` | `ANY` | The starting value of the `accumulator`.
+| `variable` | `ANY` | A variable that represents each element in the `list` during iteration.
+| `list` | `LIST<ANY>` | The `list` that is being iterated over.
+| `expression` | `ANY` | An expression that updates the `accumulator` with each iteration.
 | *Returns* 3+| `ANY`
 |===
 
-This function is analogous to the `fold` or `reduce` method in functional languages such as Lisp and Scala.
-
+.Considerations
+|===
+|`reduce()` differs from most Cypher functions because it iterates over a list, incrementally updating an accumulator with each element based on an expression, rather than returning a result from a single evaluation.
+As such, Cypher's `reduce()` is analogous to the `fold` or `reduce` methods in functional languages such as Lisp and Scala.
+|===
 
 .+reduce()+
 ======

modules/ROOT/pages/functions/predicate.adoc

@@ -53,6 +53,7 @@ CREATE
 
 .Considerations
 |===
+| `all()` differs from most Cypher functions because it iterates over a list, evaluating an expression for each element, rather than returning a result from a single evaluation.
 | `null` is returned if the `list` is `null` or if the `predicate` evaluates to `null` for at least one element and does not evaluate to false for any other element.
 | `all()` returns `true` if `list` is empty because there are no elements to falsify the `predicate`.
 |===
@@ -123,6 +124,7 @@ RETURN all(i in emptyList WHERE true) as allTrue, all(i in emptyList WHERE false
 
 .Considerations
 |===
+| `any()` differs from most Cypher functions because it iterates over a list, evaluating an expression for each element, rather than returning a result from a single evaluation.
 | `null` is returned if the `list` is `null` or if the `predicate` evaluates to `null` for at least one element and does not evaluate to false for any other element.
 | `any()` returns `false` if `list` is empty because there are no elements to satisfy the `predicate`.
 |===
@@ -348,6 +350,7 @@ xref:syntax/operators.adoc#cypher-comparison[`IS NULL` or `IS NOT NULL`] should
 
 .Considerations
 |===
+| `none()` differs from most Cypher functions because it iterates over a list, evaluating an expression for each element, rather than returning a result from a single evaluation.
 | `null` is returned if the `list` is `null`, or if the `predicate` evaluates to `null` for at least one element and does not evaluate to `true` for any other element.
 | `none()` returns `true` if `list` is empty because there are no elements to violate the `predicate`.
 |===
@@ -419,6 +422,7 @@ RETURN none(i IN emptyList WHERE true) as noneTrue, none(i IN emptyList WHERE fa
 
 .Considerations
 |===
+| `single()` differs from most Cypher functions because it iterates over a list, evaluating an expression for each element, rather than returning a result from a single evaluation.
 | `null` is returned if the `list` is `null`, or if the `predicate` evaluates to `null` for at least one element and does not evaluate to `true` for any other element.
 | `single()` returns `false` if `list` is empty because there is not exactly one element satisfying the `predicate`.
 |===