Update Windows build instructions.

docs/windows_build_instructions.md

 # Checking out and Building Chromium for Windows
 
 There are instructions for other platforms linked from the 
 [get the code](get_the_code.md) page.
 
 ## Instructions for Google Employees
 
 Are you a Google employee? See
 [go/building-chrome-win](https://goto.google.com/building-chrome-win) instead.
 
 [TOC]
 
 ## System requirements
 
 * A 64-bit Intel machine with at least 8GB of RAM. More than 16GB is highly
   recommended.
-* At least 100GB of free disk space.
+* At least 100GB of free disk space on an NTFS-formatted hard drive. FAT32
+  will not work, as some of the Git packfiles are larger than 4GB.
 * Visual Studio 2015 Update 3, see below (no other version is supported).
 * Windows 7 or newer.
 
 ## Setting up Windows
 
 ### Visual Studio
 
 As of December 8, 2016 Chromium requires Visual Studio 2015, with the 14393
 Windows SDK to build.
 
@@ skipping 17 matching lines @@
 and extract it somewhere.
 
 *** note
 **Warning:** **DO NOT** use drag-n-drop or copy-n-paste extract from Explorer,
 this will not extract the hidden “.git” folder which is necessary for
 depot_tools to autoupdate itself. You can use “Extract all…” from the 
 context menu though.
 ***
 
 Add depot_tools to the start of your PATH (must be ahead of any installs of 
-Python). Assuming you unzipped the bundle to C:\src\depot_tools:
-
-With Administrator access:
+Python). Assuming you unzipped the bundle to C:\src\depot_tools, open:
 
 Control Panel → System and Security → System → Advanced system settings
 
-Modify the PATH system variable to include C:\src\depot_tools.
+If you have Administrator access, Modify the PATH system variable and
+put `C:\src\depot_tools` at the front (or at least in front of any directory
+that might already have a copy of Python or Git).
 
-Without Administrator access:
-
-Control Panel → User Accounts → User Accounts → Change my environment variables
-
-Add a PATH user variable (or modify the existing one to include):
-`C:\src\depot_tools`.
+If you don't have Administrator access, you can add a user-level PATH
+environment variable and put `C:\src\depot_tools` at the front, but
+if your system PATH has a Python in it, you will be out of luck.
 
 Also, add a DEPOT_TOOLS_WIN_TOOLCHAIN system variable in the same way, and set
 it to 0. This tells depot_tools to use your locally installed version of Visual
 Studio (by default, depot_tools will try to use a google-internal version).
 
 From a cmd.exe shell, run the command gclient (without arguments). On first
 run, gclient will install all the Windows-specific bits needed to work with
 the code, including msysgit and python.
 
 * If you run gclient from a non-cmd shell (e.g., cygwin, PowerShell),
@@ skipping 149 matching lines @@
 $ gclient sync
 ```
 
 The first command updates the primary Chromium source repository and rebases
 any of your local branches on top of tip-of-tree (aka the Git branch `origin/master`).
 If you don't want to use this script, you can also just use `git pull` or 
 other common Git commands to update the repo.
 
 The second command syncs the subrepositories to the appropriate versions and
 re-runs the hooks as needed.