Document transparency support in buffers

[l-koehler]

If transparency is enabled, the buffer format is ARGB instead of 0RGB.
This is not reflected in the documentation, which seems to state that the format always is 0RGB, not mentioning how transparency can be added.
This is especially confusing as enabling transparency leads to nothing showing without intentionally editing the upper 8-bits, as these default to 0x00 (fully transparent).
I am using Linux/Wayland, but as transparency support isn't really a bug this shouldn't be platform related.