Clarify GlobalTransform::to_isometry doc (#19645)

# Objective

- to_isometry is not a direct conversion, it involves computation. the
docs could be clearer

## Solution

- Improve docs

## Testing

- its docs

Author: atlv24

crates/bevy_transform/src/components/global_transform.rs

@@ -139,8 +139,9 @@ impl GlobalTransform {
         }
     }
 
-    /// Returns the isometric part of the transformation as an [isometry]. Any scaling done by the
-    /// transformation will be ignored.
+    /// Computes a Scale-Rotation-Translation decomposition of the transformation and returns
+    /// the isometric part as an [isometry]. Any scaling done by the transformation will be ignored.
+    /// Note: this is a somewhat costly and lossy conversion.
     ///
     /// The transform is expected to be non-degenerate and without shearing, or the output
     /// will be invalid.