Add an alternative description of str_c (#277)

Author: paleolimbot

R/c.r

@@ -1,5 +1,10 @@
 #' Join multiple strings into a single string.
 #'
+#' Joins two or more vectors element-wise into a single character vector,
+#' optionally inserting `sep` between input vectors. If `collapse` is not `NULL`,
+#' it will be inserted between elements of the result, returning a character
+#' vector of length 1.
+#'
 #' To understand how `str_c` works, you need to imagine that you are building up
 #' a matrix of strings. Each input argument forms a column, and is expanded to
 #' the length of the longest argument, using the usual recyling rules.  The