docs: filesystems: vfs: Render method descriptions

Currently vfs.rst does not render well into HTML the method descriptions
for VFS data structures.  We can improve the HTML output by putting the
description string on a new line following the method name.

Suggested-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Tobin C. Harding <tobin@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>