docs: explain how to read the molecule rank plot

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 
 - Colocalization heatmap text now explains that proteins are selected by mean abundance (up to 40 markers), globally across samples or within each cell type.
+- The Cell recovery section now explains how to read the molecule rank plot.
 - Experiment Summary labels now use "isotype control markers" consistently.
 
 ## [0.14.0] 2026-08-25

inst/quarto/workflows/amplicon_demux/quality_metrics.qmd

@@ -137,6 +137,12 @@ close_tabset()
 
 #### Number of components
 
+The molecule rank plot below ranks the components of a sample from the most to the fewest molecules. Cells appear as
+the plateau of large components to the left, and the curve drops where cells give way to background components such as
+debris. A sharp drop means that cells are well separated from background, while a gradual slope means that the two are
+harder to tell apart. The dashed lines mark the minimum and maximum component size thresholds, and only components
+between them are kept as cells.
+
 ```{r}
 #| label: qc_metrics_molrank_plot
 #| results: 'asis'