Emphasize that /* */ is only for block comments

My perception is that most uses of /* */ in our code are for block comments, so
deemphasize them in the style guide to stop people from using them for other
kinds of comment.

Rationale:

// This is a normal comment but it wraps onto
// multiple lines because it explains something
// in quite a bit of detail.
position: relative;

is easier to read than

/* This is a normal comment but it wraps onto
multiple lines because it explains something
in quite a bit of detail */
position: relative;

This is especially the case when reading code in phabricator where there's no
syntax highlighting.