Misc doc tweaks, clarify how to configure STACK_ROOT

Author: mgsloan

doc/GUIDE.md

@@ -1787,7 +1787,7 @@ it. Here is an example:
 -}
 ```
 
-## Finding project configs, and the implicit global
+## Finding project configs, and the implicit global project
 
 Whenever you run something with stack, it needs a `stack.yaml` project file. The
 algorithm stack uses to find this is:
@@ -1818,6 +1818,15 @@ configuration. It has no impact on projects at all. Every package you install
 with it is put into isolated databases just like everywhere else. The only magic
 is that it's the catch-all project whenever you're running stack somewhere else.
 
+## Setting stack root location
+
+`stack path --stack-root` will tell you the location of the "stack root". Among
+other things, this is where stack stores downloaded programs and snapshot
+packages. This location can be configured by setting the STACK_ROOT environment
+variable or passing the `--stack-root` commandline option. It is particularly
+useful to do this on Windows, where filepaths are limited (MAX_PATH), and things
+can break when this limit is exceeded.
+
 ## `stack.yaml` vs `.cabal` files
 
 Now that we've covered a lot of stack use cases, this quick summary of
@@ -2037,11 +2046,11 @@ image:
 and then run `stack image container` and then `docker images` to list
 the images.
 
-Note that the executable will be built in the development environment 
-and copied to the container, so the dev OS must match that of the 
+Note that the executable will be built in the development environment
+and copied to the container, so the dev OS must match that of the
 container OS. This is easily accomplished using [Docker integration](docker_integration.md),
-under which the exe emitted by `stack build` will be built on the 
-Docker container, not the local OS. 
+under which the exe emitted by `stack build` will be built on the
+Docker container, not the local OS.
 
 The executable will be stored under `/usr/local/bin/<your-project>-exe`
 in the running container.