Add docs for recheck tasks

dev/tasks/tasks.yml

@@ -897,6 +897,13 @@ tasks:
       - r-pkg__src__contrib__arrow_{no_rc_r_version}\.tar\.gz
 
 {% for which in ["strong", "most"] %}
+  # strong and most used here are defined by ?tools::package_dependencies as:
+  #
+  # strong: Depends, Imports, LinkingTo
+  # most: Depends, Imports, LinkingTo, Suggests
+  #
+  # So the key difference between strong and most is whether you want to expand
+  # the reverse dependency checking to Suggests (most) or not.
   r-recheck-{{which}}:
     ci: github
     template: r/github.recheck.yml