Update social-transactions-api.md - Noting that IsHidden is ignored on CREATE

[DeSoRSS]

When true, this post will be hidden. You can't hide a post when creating it, you must submit a second transaction to hide the post.

Source reference: https://github.com/deso-protocol/core/blob/e2587671948fd02c5d4f7661508480822c7b27f3/lib/block_view_post.go#L1030