Restrict subdirectory imports to main.kcl (#7094)

Signed-off-by: Nick Cameron <nrc@ncameron.org>

docs/kcl-lang/modules.md

@@ -27,9 +27,6 @@ import increment from "util.kcl"
 answer = increment(41)
 ```
 
-Imported files _must_ be in the same project so that units are uniform across
-modules. This means that it must be in the same directory.
-
 Import statements must be at the top-level of a file. It is not allowed to have
 an `import` statement inside a function or in the body of an if‑else.
 
@@ -58,6 +55,9 @@ Imported symbols can be renamed for convenience or to avoid name collisions.
 import increment as inc, decrement as dec from "util.kcl"
 ```
 
+You can import files from the current directory or from subdirectories, but if importing from a
+subdirectory you can only import `main.kcl`.
+
 ---
 
 ## Functions vs `clone`
@@ -229,6 +229,19 @@ The final statement is what's important because it's the return value of the
 entire module. The module is expected to return a single object that can be used
 as a variable by the file that imports it.
 
+The name of the file or subdirectory is used as the name of the variable within the importing program.
+If you want to use a different name, you can do so by using the `as` keyword:
+
+```kcl,norun
+import "cube.kcl"                // Introduces a new variable called `cube`.
+import "cube.kcl" as block       // Introduces a new variable called `block`.
+import "cube/main.kcl"           // Introduces a new variable called `cube`.
+import "cube/main.kcl" as block  // Introduces a new variable called `block`.
+```
+
+If the filename includes hyphens (`-`) or starts with an underscore (`_`), then you must specify a
+variable name.
+
 ---
 
 ## Multiple instances of the same import