Fix links to secondary index configuration in 2.8 release notes

Change-Id: I15cd19510965f3dda4931c46d7a4ce25eba16756
Add warning about secondary index in the 2.8 release notes

Add a warning that after enabling the secondary index the index must
be built before restarting Gerrit.

This is already mentioned in the configuration documentation, but it
doesn't hurt to also mention it in the release notes.

Change-Id: I34de772112396f0863225d6ac50aff04698fc6e4