More descriptive NEWS item and added an explanatory comment to the code

Author: bborgesr

NEWS.md

@@ -15,7 +15,7 @@ shiny 1.0.3.9000
 
 ### Bug fixes
 
-* Fixed [#1546](https://github.com/rstudio/shiny/issues/1546): make it possible (without any hacks) to write arbitrary data into a module's `session$userData` (which is exactly the same environment as the parent's `session$userData`). ([#1732](https://github.com/rstudio/shiny/pull/1732)).
+* Fixed [#1546](https://github.com/rstudio/shiny/issues/1546): make it possible (without any hacks) to write arbitrary data into a module's `session$userData` (which is exactly the same environment as the parent's `session$userData`). To be clear, it allows something like `session$userData$x <- TRUE`, but not something like `session$userData <- TRUE` (that is not allowed in any context, whether you're in the main app, or in a module) ([#1732](https://github.com/rstudio/shiny/pull/1732)).
 
 * Fixed [#1701](https://github.com/rstudio/shiny/issues/1701): There was a partial argument match in the `generateOptions` function. ([#1702](https://github.com/rstudio/shiny/pull/1702))
 

R/modules.R

@@ -26,6 +26,10 @@ createSessionProxy <- function(parentSession, ...) {
 
 #' @export
 `$<-.session_proxy` <- function(x, name, value) {
+  # this line allows users to write into session$userData
+  # (e.g. it allows something like `session$userData$x <- TRUE`,
+  # but not `session$userData <- TRUE`) from within a module
+  # without any hacks (see PR #1732)
   if (identical(x[[name]], value)) return(x)
   stop("Attempted to assign value on session proxy.")
 }