logging: minor clean up in Logger.h
logging: minor clean up in Logger.h

Summary:
Update the comment describing the Logger class, and remove a `clang-format off`
since recent versions of `clang-format` appear to do a good job formatting this
macro.

Reviewed By: yfeldblum

Differential Revision: D6525996

fbshipit-source-id: 6abbb3740afb53feb2418ca890cd78af1eb5f7a2

facebook/folly,rklabs/folly,facebook/folly,rklabs/folly,facebook/folly,rklabs/folly,rklabs/folly,facebook/folly,facebook/folly,rklabs/folly