Document the need for disabling BB if building without network access (#38918)

* Document the need for disabling BB if building without network access

Fix #33569

* Update doc/build/build.md

Co-authored-by: Dilum Aluthge <dilum@aluthge.com>

Co-authored-by: Keno Fischer <keno@juliacomputing.com>
Co-authored-by: Dilum Aluthge <dilum@aluthge.com>

Author: 3 people

doc/build/build.md

@@ -17,8 +17,8 @@ variables.
 When compiled the first time, the build will automatically download
 pre-built [external
 dependencies](#required-build-tools-and-external-libraries). If you
-prefer to build all the dependencies on your own, add the following in
-`Make.user`
+prefer to build all the dependencies on your own, or are building on a system that cannot
+access the network during the build process, add the following in `Make.user`:
 
 ```
 USE_BINARYBUILDER=0