docs: path-lookup: markup fixes for emphasis

Underscores were being used for emphasis, but these are rendered verbatim
in HTML output. reStructuredText uses asterisks for emphasis. I *think* I
caught all of them.

Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Link: https://lore.kernel.org/r/20200727121525.28103-2-vegard.nossum@oracle.com


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