docs: exportfs: Use source code struct documentation

docs: exportfs: Use source code struct documentation

Instead of duplicating struct export_operations documentation in both
ReST file and in the C source code, just use the kernel-doc in the docs.

While here, make the sentence preceding the paragraph less redundant.

Signed-off-by: André Almeida <andrealmeid@igalia.com>
Link: https://patch.msgid.link/20260112-tonyk-fs_uuid-v1-4-acc1889de772@igalia.com
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>