How can I add a comment for each flag, on separate lines, in a bash script?

You can evaluate comments before the slash, simply generating empty strings for comments which won't affect your script:

bwa aln \
-n $n `# -n max #diff (integer) or missing prob under 0.02 err rate (float) [0.04]` \
-o $o `# -o maximum number or fraction of gap opens [1]` \
....

I suggest using multiple lines to comment the code before the command. like

 # this command uses multiple parameters
 # it requires 4 parameters and none are optional with no defaults
 # parameters used : 
 # -q              name of the file
 # -b              size to truncate
 # -n              new location
 # -r              recursive

One option is to construct your command in pieces so you don't have to worry about line continuation:

cmd='date'                # run the date program
cmd=${cmd}' -d 20130905'  #  for this date
cmd=${cmd}' +%s'          #  with output in this format
echo $cmd                 # review the command
eval $cmd                 # run the command