include: update uv_is_active() documentation
include: update uv_is_active() documentation

Seems to have been overlooked in the reference counting refactor back
in May 2012 in commit 9efa8b3.  Clarify what "active" means for
different kinds of handles.

unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso