fix#95BeagleY-USB-C-power-description-wrong

This update corrects USB-C power documentation inaccuracies and aligns functionality with implementation:

1. Power Corrections: Removes false PD negotiation claims; clarifies 5.1kΩ resistors signal UFP role (power sink) and mandates user-verified 3A sourcing.
2. Functionality Fixes: Replaces MTP with USB Mass Storage (UMS), validated via driver code (drivelistmodel.cpp).
3. Risks: Warns of instability/port damage if undersourced and specifies 15W (5V@3A) power requirements.