docs: add module hierarchy section (#22919)

doc/docs.md

@@ -87,6 +87,7 @@ by using any of the following commands in a terminal:
 
 * [Module imports](#module-imports)
     * [Selective imports](#selective-imports)
+    * [Module hierarchy](#module-hierarchy)
     * [Module import aliasing](#module-import-aliasing)
 * [Statements & expressions](#statements--expressions)
     * [If](#if)
@@ -1577,6 +1578,54 @@ println('Name: ${name}')
 current_os := user_os()
 println('Your OS is ${current_os}.')
 ```
+### Module hierarchy
+
+> [!NOTE]
+> This section is valid when .v files are not in the project's root directory.
+
+Modules names in .v files, must match the name of their directory.
+
+A .v file `./abc/source.v` must start with `module abc`. All .v files in this directory 
+belong to the same module `abc`. They should also start with `module abc`.
+
+If you have `abc/def/`, and .v files in both folders, you can `import abc`, but you will have 
+to `import abc.def` too, to get to the symbols in the subfolder. It is independent.
+
+In `module name` statement, name never repeats directory's hierarchy, but only its directory.
+So in `abc/def/source.v` the first line will be `module def`, and not `module abc.def`.
+
+`import module_name` statements must respect file hierarchy, you cannot `import def`, only
+`abc.def`
+
+Refering to a module symbol such as a function or const, only needs module name as prefix:
+
+```v ignore
+module def
+
+// func is a dummy example function.
+pub fn func() {
+	println('func')
+}
+```
+
+can be called like this:
+
+```v ignore
+module main
+
+import def
+
+fn main() {
+	def.func()
+}
+```
+
+A function, located in `abc/def/source.v`, is called with `def.func()`, not `abc.def.func()`
+
+This always implies a *single prefix*, whatever sub-module depth. This behavior flattens 
+modules/sub-modules hierarchy. Should you have two modules with the same name in different
+directories, then you should use Module import aliasing (see below).
+
 
 ### Module import aliasing