adding docs for new output.devtoolNamespace option and [namespace] placeholder

Author: MagicDuck

src/content/configuration/output.md

@@ -165,6 +165,12 @@ devtoolModuleFilenameTemplate: info => {
 
 If multiple modules would result in the same name, [`output.devtoolFallbackModuleFilenameTemplate`](#output-devtoolfallbackmodulefilenametemplate) is used instead for these modules.
 
+## `output.devtoolNamespace`
+
+`string`
+
+This option determines the modules namespace used with the [`output.devtoolModuleFilenameTemplate`](#output-devtoolmodulefilenametemplate). When not specified, it will default to the value of: [`output.library`](#output-library). It's use is to prevent source file path collisions in sourcemaps when loading multiple libraries built with webpack.
+For example, if you have 2 libraries, with namespaces `library1` and `library2`, which both have a file `./src/index.js` (with potentially different contents), they will expose these files as `webpack://library1/./src/index.js` and `webpack://library2/./src/index.js`.
 
 ## `output.devtoolNamespace`