In this DOCS reference: http://docs.mongodb.org/v2.4/core/index-creation/

Please amend the DOC to add clarity on index build behaviour, in particular the following statements:

In version 2.4, a foreground index build is default, when complete on a replica set primary, it'll replicate to the secondaries with a foreground build task (regardless if the primary was issued a foreground or background index option). The replication worker will take a global DB lock, which will queue reads and writes to all DBs on that server.