Improve spatial registration guide explanation of annotation

[lahuuki]

The annotate-cell-types-by-top-correlation section of the Guide to spatial registration vignette is very simple and sparse.

This section could use some additional details about how the annotation works, the associated parameters, and how to interpret the outcome/plot. Especially noting that these annotations are meant to guide and are not statistically guaranteed.

[lcolladotor]

(private) Zoom recording info: https://jhu-genomics.slack.com/archives/C071A1RSDRD/p1749139202690569