include: document barrier functions
include: document barrier functions

Add some basic doc comments to the uv_barrier_*() functions and document
the "serializer" return value from the previous commit.

Signed-off-by: Fedor Indutny <e39d94bae70283f13d24c278e04952d6dad9384b@indutny.com>

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