About comments :

General purpose comments in the top of your file :

If you think this is required you can add a general purpose comment in the top of your file, for that, use the SCSS single line comment, so they are not added to the css file :
// These comments are in single line
// They will not appear in the CSS output,
// since they use the single-line comment syntax.
.ga-button {

Supports comment :

You should avoid support comments, but it is possible when required to add comments on code blocks to give information about what is left to do or more importantly what is here for legacy support purpose only.
Please consider that the TODOS are not a good practice and that using the project management tool for that is way better. Still, I have no religion about that.
For this type of comment, use native CSS comments Add the capitalized flags LEGACY_SUPPORT and END_LEGACY_SUPPORT at the start and at the end of the legacy support code block. Add also the version in witch the support is supposed to be dropped.
.go-my-block {
/* LEGACY SUPPORT : will be removed in version 2 */
&__toto {
// replace .go-my-block__toto in version 1.9.6 and higher
&__tata {
/* TODO: do something (this should be avoided in production) */

Code comment :

Obviously use as much code comments as you want. Use SCSS comments for that.
.ga-my-block {
flex: 1 0 0; // do not use abreviation for ie 10