embedder: fix bit-order in software pixel format description (#57156)

The order of the components for packed software pixel formats is incorrectly documented as being the order in the native type, least-significant-bit first. In reality it's the other way around. For example, for `RGB565`, the `R` is the 5 most significant bits in the 2-byte pixel value, rather than the least significant bits. The test even verify it is that way:

https://github.com/flutter/engine/blob/main/shell/platform/embedder/tests/embedder_unittests.cc#L2782-L2785

I assume noone used the software pixel formats until @sodiboo did, that's why it's gone unnoticed for so long.

Also contains some other minor documentation improvements.

- Issue: https://github.com/flutter/flutter/issues/160149

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style