[docs] Add info about using focal / anchor points (#3614)

## Description

As stated in #3611, docs are a bit unclear when it comes to explaining
usage of `anchor` and `focal` point in callbacks. Hopefully after this
PR they'll be less confusing.

## Test plan

Read docs 😄

Author: m-bert

packages/docs-gesture-handler/docs/gestures/pinch-gesture.md

@@ -93,6 +93,10 @@ const styles = StyleSheet.create({
 });
 ```
 
+## Remarks
+
+- When implementing pinch based on `focal` point, make sure to use it after gesture has activated, i.e. in `onStart`, `onUpdate` or `onChange` callbacks. Using it in `onBegan` may lead to unexpected behavior.
+
 ## Config
 
 <BaseEventConfig />

packages/docs-gesture-handler/docs/gestures/rotation-gesture.md

@@ -90,6 +90,10 @@ const styles = StyleSheet.create({
 });
 ```
 
+## Remarks
+
+- When implementing rotation based on `anchor` point, make sure to use it after gesture has activated, i.e. in `onStart`, `onUpdate` or `onChange` callbacks. Using it in `onBegan` may lead to unexpected behavior.
+
 ## Config
 
 <BaseEventConfig />