Enough work has been done on this topic, and Octave coding guidelines no longer specify a maximum line length.  Closing report.

I fixed the long @seealso lines in the scripts and libinterp directories in this changeset: http://hg.savannah.gnu.org/hgweb/octave/rev/81d26e8481a6.

They can be split, but in the past we didn't do it because each line represented a single calling form for the function which seemed like one continuous idea.

I don't think I would change the @deftypefn, but I would look at possibly splitting the @seealso links.  Those can also get very long.  Would need to check that this doesn't break munge-texi.pl or mk-doc-cache.pl.

The Texinfo manual says

'@deftypefn CATEGORY DATA-TYPE NAME ARGUMENTS...'
     The '@deftypefn' command is the general definition command for
     functions and similar entities that may take arguments and that are
     typed.  The '@deftypefn' command is written at the beginning of a
     line and is followed on the same line by the category of entity
     being described, the type of the returned value, the name of this
     particular entity, and its arguments, if any.

so it appears that it must be on one line.

Don't think there is a concensus yet on the best line length to use.

New file uploaded with correct filenames.

(file #48230)

I'm not sure what happened, but the output attached here shows lines from mx-op-defs.h with the name child-list.cc.  Similarly, lines from Faddeeva.cc with the name blas-xtra/xerbla.cc.