raft: add comments to raft_ev.h declarations

Our checkpatch.pl script demands that every new function has a comment.
One new function will be added to raft_ev.h in the following commits,
and, according to the new rules, it has to have a comment.

Having one function in a file commented looks stupid, so let's add
comments to every such function in raft_ev.h

NO_DOC=making lint happy
NO_CHANGELOG=making lint happy
NO_TEST=making lint happy