libdw: Update dwarf_cu_dwp_section_info documentation

libdw: Update dwarf_cu_dwp_section_info documentation

Update the documentation of dwarf_cu_dwp_section_info to make clear
that the function only returns an error if the DWARF package file data
couldn't be read or an unknown section constant is provided.  Missing
DWP information for a given CU isn't an error and will set both OFFSET
and SIZE to zero.  It also makes sure the documentation is < 76 chars
wide.

	* libdw/libdw.h (dwarf_cu_dwp_section_info): Update docs.

Signed-off-by: Mark Wielaard <mark@klomp.org>