'As for' means 'with regards to', or 'concerning', or 'With respect to'. Eg. "Concerning the writing itself .... ", "With regards the writing itself ..." etc.

it means the writing was corrected as needed, the grammar and spelling mistakes that were needed were corrected, and also the proposal as been annotated, where the writer personally thought annotation was needed. As has many definitions, this particular use doesn't seem to be explained in many dictionaries. You could apply this sentence to all corrections on italki, that/this is what the correctors do. They correct the spelling and grammar and to varying degrees they annotate with extra comments, tips, suggestions etc where they personally think they are needed.