[fsp] Publish documentation for writable operations.

Writable operations are completed and pretty stable, so it's time to let
developers know about them and start using them!

TEST=Confirmed on a testing doc server.
BUG=248427

Review URL: https://codereview.chromium.org/583893002

Cr-Commit-Position: refs/heads/master@{#295838}
1 file changed