website: clarify `file()` base location

refs #5213

website/source/docs/configuration/interpolation.html.md

@@ -129,7 +129,11 @@ The supported built-in functions are:
 
   * `file(path)` - Reads the contents of a file into the string. Variables
       in this file are _not_ interpolated. The contents of the file are
-      read as-is.
+      read as-is. The `path` is interpreted relative to the working directory.
+      [Path variables](#path-variables) can be used to reference paths relative
+      to other base locations. For example, when using `file()` from inside a
+      module, you generally want to make the path relative to the module base,
+      like this: `file("${path.module}/file")`.
 
   * `format(format, args...)` - Formats a string according to the given
       format. The syntax for the format is standard `sprintf` syntax.