SERVER-13267 change checkForIdIndexes recommendation to dochub link

(cherry picked from commit 502bb27fae52b9d6a38be9e51e20ac82d8c10f00)
SERVER-13268 minor grammatical cleanup of stage_builder.cpp warnings

(cherry picked from commit 984fe4b418db3341132026050a446d8c64639881)
SERVER-13269 general log message improvements for 2.6

(cherry picked from commit c27e46eceedfd68801ae79af4a5bf87b86a6319d)