Fix inconsistent return type documentation for predicate functions (toss#1540)

* Initial plan

* Fix predicate function return type documentation for consistency

Change Returns section from type predicate syntax (e.g., `value is string`)
to `boolean` for all predicate functions across all 4 languages (en, ko, ja, zh_hans).

Co-authored-by: dayongkr <[email protected]>

* Revert incorrect changes, only fix isLength.md return type mismatch

Reverts changes that incorrectly changed type predicate returns to boolean.
Only isLength needs to have boolean return type (matching source code).
Other predicate functions should keep their type predicate returns.

Co-authored-by: dayongkr <[email protected]>

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: dayongkr <[email protected]>

Author: Copilot

docs/ja/reference/predicate/isLength.md

@@ -45,4 +45,4 @@ function processLength(value: unknown) {
 
 #### 戻り値
 
-(`value is number`): 値が有効な長さの場合は `true`、そうでなければ `false` を返します。
+(`boolean`): 値が有効な長さの場合は `true`、そうでなければ `false` を返します。

docs/ko/reference/predicate/isLength.md

@@ -45,4 +45,4 @@ function processLength(value: unknown) {
 
 #### 반환 값
 
-(`value is number`): 값이 유효한 길이면 `true`, 그렇지 않으면 `false`를 반환해요.
+(`boolean`): 값이 유효한 길이면 `true`, 그렇지 않으면 `false`를 반환해요.

docs/reference/predicate/isLength.md

@@ -45,4 +45,4 @@ function processLength(value: unknown) {
 
 #### Returns
 
-(`value is number`): Returns `true` if the value is a valid length, `false` otherwise.
+(`boolean`): Returns `true` if the value is a valid length, `false` otherwise.

docs/zh_hans/reference/predicate/isLength.md

@@ -45,4 +45,4 @@ function processLength(value: unknown) {
 
 #### 返回值
 
-(`value is number`): 如果值为有效长度则返回 `true`，否则返回 `false`。
+(`boolean`): 如果值为有效长度则返回 `true`，否则返回 `false`。