docs: filesystems: Add mount map description in Content

The second paragraph of the content section does not properly
describe how mount points are determined by autofs.

Replace the lines detailing how the determination of these mount
points is "ad hoc" by a short description of the mount map syntax
used by autofs.

Signed-off-by: Jaskaran Singh <jaskaransingh7654321@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>