LUDOC-117 style: use <replaceable> in example text

LUDOC-117 style: use <replaceable> in example text

Use the <replaceable> tag and {} instead of <emphasis> and
<> for user-supplied text.  The XML markup is intended to
be semantic, so <replaceable> is more correct to use.

The use of <> makes the XML ugly, since it needs to be
written as &gt;&lt; in the .xml files.

Some incorrect usages of <emphasis> were corrected.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Richard Henwood <richard.henwood@intel.com>
Change-Id: I8edb59a100e20573e06f11ce10fe1642d45fbd4f
Reviewed-on: http://review.whamcloud.com/5006
Tested-by: Hudson