docs: path-lookup: more markup fixes

"xxx``at``" makes the `` appear verbatim in the HTML output. I've opted
for changing this into ``*at()`` to harmonise this with the use of * seen
later in the same document (and add the parentheses to clarify that this
is a system/function call).

``path_``* also makes `` appear in the HTML output, but we can fix it by
moving the * into the ``. Also add the parantheses here.

Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Link: https://lore.kernel.org/r/20200727121525.28103-1-vegard.nossum@oracle.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>