doc: llext: add extension bringup and teardown documentation

Add documentation for the new LLEXT APIs that allow to call the
initialization and cleanup functions of an extension.

Signed-off-by: Luca Burelli <[email protected]>

Author: pillo79

doc/services/llext/load.rst

@@ -24,6 +24,32 @@ successfully, the extension is loaded into memory and is ready to be used.
    included in any user memory domain. To allow access from user mode, the
    :c:func:`llext_add_domain` function must be called.
 
+Initializing and cleaning up the extension
+==========================================
+
+The extension may define a number of initialization functions that must be
+called after loading but before any function in it can be used; this is typical
+in languages such as C++ that provide the concept of object constructors. The
+same is true for cleanup functions that must be called before unloading the
+extension.
+
+LLEXT supports calling the functions listed in the ``.preinit_array`` and
+``.init_array`` sections of the ELF file with the :c:func:`llext_bringup`
+function, and the functions listed in the ``.fini_array`` section with the
+:c:func:`llext_teardown` function. These APIs are compatible with
+:ref:`User Mode <usermode_api>`, and thus can be called from either kernel or
+user context.
+
+.. important::
+   The code run by these functions is fully determined by the contents of the
+   ELF file. This may have security implications if its origin is untrusted.
+
+If the extension requires a dedicated thread, the :c:func:`llext_bootstrap`
+function can be used to minimize boilerplate code. This function has a
+signature that is compatible with the :c:func:`k_thread_create` API, and will
+call :c:func:`llext_bringup`, then a user-specified function in the same
+context, and finally :c:func:`llext_teardown` before returning.
+
 Accessing code and data
 =======================