doc-rst: FE_READ_SIGNAL_STRENGTH: improve man-like format

Parsing this file were causing lots of warnings with sphinx,
due to the c function prototypes.

Fix that by prepending them with .. c:function::

While here, use the same way we document man-like pages,
at the V4L side of the book and add escapes to asterisks.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>