Clarify crossing factor level behavior

NEWS.md

@@ -1,5 +1,8 @@
 # tidyr (development version)
 
+* The `crossing()` documentation now clarifies that factor inputs use the full
+  set of levels, not just the values present in the data (#1526).
+
 * `chop()` gains a `by` argument for specifying grouping columns, similar to `nest(.by =)` (@hrryt, #1490).
 
 * Specifying `chop()`'s `cols` argument by position is soft-deprecated. It must instead be specified by name, which better communicates intent now that `chop()` also has a `by` argument (#1490).

R/expand.R

@@ -33,9 +33,9 @@
 #'     a row for each present school-student combination for all possible
 #'     dates.
 #'
-#'   When used with factors, [expand()] and [complete()] use the full set of
-#'   levels, not just those that appear in the data. If you want to use only the
-#'   values seen in the data, use `forcats::fct_drop()`.
+#'   When used with factors, [expand()], [complete()], and [crossing()] use the
+#'   full set of levels, not just those that appear in the data. If you want to
+#'   use only the values seen in the data, use `forcats::fct_drop()`.
 #'
 #'   When used with continuous variables, you may need to fill in values
 #'   that do not appear in the data: to do so use expressions like