You can either make the change in place, in /usr/bin/apropos, or you can place a copy of the original somewhere earlier in your path and edit the copy. The modification is placed at the very end of the script, and involves inserting the command cut -c1-80 | into the pipeline, immediately before the $PAGER invocation. Here is the resulting line:
( printf -- "$line2"; echo "$line"; cat ) | cut -c1-80 | $PAGER
What the modification does is to truncate all characters to the right of column 80. The change will also truncate summary lines that are valid but long -- if you think more data should be written out, change the 80 to whatever you prefer.
[robg adds: I tested this by first backing up the original apropos, and then editing it with sudo vi apropos. I changed the last line and then saved the file (I had to use w! in vi to override the read-only flag). It worked as described; apropos serial now produces reasonable output...]

