Update building from source docs

Author: keyprocedure

docs/source/using-executorch-building-from-source.md

@@ -61,7 +61,7 @@ Or alternatively, [install conda on your machine](https://conda.io/projects/cond
    # Install ExecuTorch pip package and its dependencies, as well as
    # development tools like CMake.
    # If developing on a Mac, make sure to install the Xcode Command Line Tools first.
-   # Intel-based macOS systems require building PyTorch from source (see below)
+   # Intel-based macOS systems require building PyTorch, Torchvision, and Torchaudio from source (see below)
    ./install_executorch.sh
    ```
 

docs/source/using-executorch-ios.md

@@ -114,7 +114,7 @@ python3 -m venv .venv && source .venv/bin/activate && pip install --upgrade pip
 ./backends/apple/coreml/scripts/install_requirements.sh
 ```
 
-- **Intel-based macOS** systems require building PyTorch from source:  
+- **Intel-based macOS** systems require building PyTorch, Torchvision, and Torchaudio from source:  
   - Use the [`--use-pt-pinned-commit` flag](https://github.com/pytorch/executorch/blob/main/install_requirements.sh) to install Executorch with an existing PyTorch build.  
     See the [PyTorch instructions](https://github.com/pytorch/pytorch#installation) on how to build PyTorch from source.
     ```bash