Clarify Sass docs

[cpsievert]

For instance, for bs_add_rules(theme, rules), what are rules (a character vector)?

[russHyde]

Would you need any help with this issue? I'm currently learning my way around bslib and sass and would be happy to help with docs

[cpsievert]

That'd be great! Note that in most of these instances, we'll want to point to sass::as_sass(), and some cases sass::sass_bundle()/sass::sass_layer()