Clarify Web Platform Support for DisplayServer.virtual_keyboard_get_height()

**Your Godot version:**
Godot v4.4.1.stable.official

**Issue description:**
When exporting to the web (tested on iPhone 12), the `DisplayServer.virtual_keyboard_get_height()` function returns: *"Virtual keyboard not supported by this display server."*

While other `DisplayServer.virtual_keyboard_*` functions seem to work correctly, this particular method appears to be unsupported on web platforms. If this is a known limitation, the documentation should clarify that it is only available on native platforms (iOS, Android). Otherwise, if this behavior is unexpected, I will open a separate issue.

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/classes/class_displayserver.html#class-displayserver-method-virtual-keyboard-get-height