doc: add return type to Blocklist.isBlocklist

Signed-off-by: Yuki Motokane <216834584+motokaneyuki@users.noreply.github.com>

Author: motokaneyuki

doc/api/net.md

@@ -248,7 +248,7 @@ added:
 -->
 
 * `value` {any} Any JS value
-* Returns `true` if the `value` is a `net.BlockList`.
+* Returns {boolean} `true` if the `value` is a `net.BlockList`.
 
 ### `BlockList.PRIVATE_RANGES`