docs: document that linkedSignal with previous should include type arguments (angular#60857)

Due to the cyclic nature of `previous`'s type and the computation's return type,
TypeScript isn't able to infer generic type arguments.

Closes angular#60423

PR Close angular#60857

Author: JoostK

adev/src/content/guide/signals/linked-signal.md

@@ -108,6 +108,8 @@ The `source` can be any signal, such as a `computed` or component `input`. When
 
 The `computation` is a function that receives the new value of `source` and a `previous` object. The `previous` object has two properties— `previous.source` is the previous value of `source`, and `previous.value` is the previous result of the `computation`. You can use these previous values to decide the new result of the computation.
 
+HELPFUL: When using the `previous` parameter, it is necessary to provide the generic type arguments of `linkedSignal` explicitly. The first generic type corresponds with the type of `source` and the second generic type determines the output type of `computation`.  
+
 ## Custom equality comparison
 
 `linkedSignal`, as any other signal, can be configured with a custom equality function. This function is used by downstream dependencies to determine if that value of the `linkedSignal` (result of a computation) changed: