doc: fuse: Add comments and bring into API docs

Provide some comments for most structs (and some #defines) and include
the file in the API docs so it is checked.

Signed-off-by: Simon Glass <sjg@chromium.org>

doc/api/fuse.rst

@@ -0,0 +1,8 @@
+.. SPDX-License-Identifier: GPL-2.0+
+.. Copyright 2025 Simon Glass <sjg@chromium.org>
+
+FUSE - Filesystem in Userspace
+==============================
+
+.. kernel-doc:: include/linux/fuse.h
+   :internal:

doc/api/index.rst

@@ -13,6 +13,7 @@ U-Boot API documentation
    efi
    event
    fs
+   fuse
    getopt
    interrupt
    led