Improve documentation on figure labels for cross-referencing

[tklebel]

Transitioning from RMarkdown, it took me ages to understand that the labels for figures always need to have the prefix fig- for the figure numbering to work

This works:

#| label: fig-plots
#| fig-cap: "First figure."
plot(1:5)

This does not:

#| label: test-fig
#| fig-cap: "Second figure."
plot(1:6)

I was treating the labels similar to chunk labels in RMarkdown, and although the examples use the prefix throughout, the top paragraph on the docs only mentions a unique label, not a unique label that is prefixed with fig-:

Cross-references make it easier for readers to navigate your document by providing numbered references and hyperlinks to various entities like figures and tables. Every cross-referenceable entity requires a label (unique identifier) and caption (description).

It is somewhat mentioned in the "note" box below, but imho this is too low down in the text. Maybe the latter sentence could be amended with "requires a label (unique identifier prefixed with fig-) and caption..."?

[jjallaire]

Thanks for pointing this out! Addressed here: d39c977

[tklebel]

Awesome, thanks!