Define what all input functions yield for server (#2600)

* adding section to roxygen docs for server value of input funcs

* add more server values to roxygen docs

* add more server values

* update more server values in documentation

* Address PR feedback

I reflowed the comments to have a max width of 80 on some files, so there were a flurry of line changes, but the only substantive changes were around Joe's comments.

* Update R/input-slider.R

Co-Authored-By: Hadley Wickham <h.wickham@gmail.com>

* Apply suggestions from code review

Co-Authored-By: Carson Sievert <cpsievert1@gmail.com>

* PR feedback, regenerate docs.

* PR Feedback

* Eliminate extra newline

* passwordInput()'s server value

* varSelectInput()'s server value

* document

R/input-text.R

@@ -28,6 +28,11 @@
 #' }
 #' shinyApp(ui, server)
 #' }
+#'
+#' @section Server value:
+#' A character string of the text input. The default value is `""`
+#' unless `value` is provided.
+#'
 #' @export
 textInput <- function(inputId, label, value = "", width = NULL,
   placeholder = NULL) {