clarify documentation for IOBuf::length()
clarify documentation for IOBuf::length()

Summary:
The documentation on length() is somewhat ambiguous
when dealing with chains of IOBufs -- it is only clear after
reading implementations that "data" refers to "the data in
this single IOBuf, potentially only part of a chain".

Reviewed By: yfeldblum

Differential Revision: D15830189

fbshipit-source-id: d3319abd2d53a76665442d1b93b78a7d1a77dcc0

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