Improve layer documentation

[teunbrand]

This PR aims to fix #4770, fix #5040 and fix #4606.

Briefly, it does the following things:

• It makes 3 standalone documented topics for the layer geom, layer stat and layer position arguments.
• It expands the description of these arguments in layer() and links them to the standalone topics.
• The ... argument to geom_*() and stat_*() functions is now more explicit about what it accepts and rejects.