coding style: Change the comment style in argument.c

 Instead of having two practically-useless lines, one with "/*" at
the start of the block comment, and one with "*/" at the end of the
block comment, just have one at the end, and immediately start the
block comment at the opening line.

 This makes it more readable and less inefficient, especially for
one-line comments (which will get changed in the next commit).
coding style: Change one-line comment style in argument.c

 For one-line comments, still use block comment syntax, but don't
include the extra line at the bottom to close the comment; do the
entire comment on one line.