user guide: document an Environment a bit more

doc/user_guide.md

@@ -369,13 +369,32 @@ source files found recursively under the `src` directory.
 
 ### Environments
 
-An Environment is a collection of:
+An Environment includes:
 
-  - construction variables
+  - a collection of construction variables
-  - build hooks
+  - a collection of build hooks
-  - registered build targets
+  - a collection of user-registered build targets
+  - a build root
 
 All build targets must be registered within an `Environment`.
+The Environment's build root is a directory created within the top-level
+Rscons build directory.
+By default it holds all intermediate files generated by Rscons that are needed
+to produce a user-specified build target.
+For example, for the `Rsconscript`:
+
+```ruby
+build do
+  Environment.new do |env|
+    env.Program("myprog.exe", glob("src/**/*.c"))
+  end
+end
+```
+
+Rscons will place an object file and dependency file corresponding to each C
+source file under the Environment's build root.
+This keeps the intermediate generated build artifacts separate from the source
+files.
 
 ### Specifying Source Files: The glob Method