Fix W2Vtrace definition in documentation

### Are there any linked Issues or Pull Requests?

_No response_

### Brief description

The W2trace definitions are not consistent with each other in terms of their scalar or vector state. There is the additional question as to whether they should be implemented as vector or scalar but this issue is just aimed at making the docs correct.

https://github.com/MetOffice/lfric_core/blob/bf236737858c4ed29957124a2f64fd0e4f4fd7be/documentation/source/how_it_works/core_data_model/science_model_architecture.rst?plain=1#L727-L735

### Further details of the issue.

_No response_

Read documentation/source/how_it_works/core_data_model/science_model_architecture.rst around lines 727–735 and compare the W2trace definitions there. Update the documentation so the scalar or vector state is described consistently; the implementation choice is explicitly outside this issue, and the section should be internally correct when finished.

Written by the indexing model from the issue text.