docs: update array value docs to reflect RediSearch-compatible error behavior

Remove element-wise operations, array-specific functions, and broadcasting.
Arrays now return errors/nil/nan when passed to scalar functions or arithmetic,
matching RediSearch behavior.

Signed-off-by: Alexandru Filip <alexandru.filip@improving.com>

Author: AlexFilipImproving

topics/search-expressions.md

@@ -178,55 +178,28 @@ REDUCE TOLIST 1 <expression>
 
 Arrays can be nested — an array may contain other arrays as elements, enabling multi-dimensional data structures within the expression evaluation context.
 
-## Element-wise Scalar Functions
+## Scalar Functions on Arrays
 
-When a scalar function receives an array argument, it applies independently to each element, returning a new array of the same length.
+Scalar functions do not apply element-wise to arrays. Passing an array to a scalar function produces a nil or nan result, consistent with RediSearch behavior.
 
-| Function | Description |
-| :------- | :---------- |
-| `lower(array)` | Applies lower-case conversion to each element |
-| `upper(array)` | Applies upper-case conversion to each element |
-| `strlen(array)` | Returns string length of each element |
-| `floor(array)` | Applies floor to each element |
-| `ceil(array)` | Applies ceiling to each element |
-| `abs(array)` | Applies absolute value to each element |
-| `log(array)` | Applies natural log to each element |
-| `sqrt(array)` | Applies square root to each element |
+Functions that return nil when applied to an array: `lower`, `upper`, `strlen`, `startswith`, `contains`.
 
-## Array Arithmetic
-
-When both operands are arrays of equal length, arithmetic operators apply element-wise, producing a new array where each element is the result of applying the operator to the corresponding elements of the input arrays.
-
-When one operand is a scalar and the other is an array, the scalar is broadcast — promoted to an array of the same length — so the operation applies to each element.
+Functions that return nan when applied to an array: `floor`, `ceil`, `abs`, `log`, `log2`, `exp`, `sqrt`.
 
-| Operator | Operation |
-| :------: | :-------- |
-| `+` | Element-wise addition |
-| `-` | Element-wise subtraction |
-| `*` | Element-wise multiplication |
-| `/` | Element-wise division |
-| `^` | Element-wise exponentiation |
-
-## Array-Specific Functions
+## Array Arithmetic
 
-| Syntax | Description |
-| :----- | :---------- |
-| `arraylen(array)` | Returns the number of elements in the array |
-| `arrayat(array, index)` | Returns the element at the zero-based index |
-| `isarray(value)` | Returns 1 if the value is an array, otherwise 0 |
-| `makearray(e1, e2, ...)` | Constructs an array from the provided arguments |
-| `flatten(array, depth)` | Flattens nested arrays to the specified depth |
+Any mathematical operation (`+`, `-`, `*`, `/`, `^`) involving an array operand returns the error `"Could not convert value to a number"`. This is consistent with RediSearch behavior.
 
 ## Error Handling
 
-Array operations produce specific error messages when encountering invalid conditions.
+Array operations produce specific error messages when encountering invalid conditions, consistent with RediSearch error behavior.
 
-| Condition | Message Format |
-| :-------- | :------------- |
-| Arithmetic with incompatible type | `Type error: cannot add array to string` |
-| Element-wise on mismatched lengths | `Length mismatch: arrays have lengths 3 and 5` |
-| Per-element computation failure | `Element error at index 2: division by zero` |
-| `arrayat` out-of-bounds | `Index out of bounds: index 5, array length 3` |
+| Condition | Result |
+| :-------- | :----- |
+| Arithmetic with array operand | `"Could not convert value to a number"` |
+| `substr` with array argument | `"Invalid type for substr. Expected string"` |
+| String functions on array | nil (empty value) |
+| Numeric functions on array | nan |
 
 ## RESP Serialization