Clarify `IStorageSession`'s `WOULD_BLOCK` behavior

Add an explanation to `IStorageSession`'s doc comment of how the
interface uses `WOULD_BLOCK` error returns.

Test: none
Bug: 278779487
Change-Id: Ib2fa6e5bdf594f62370f2eb8852cf76f1d0c7dde
1 file changed