sync: Provide package documentation

Briefly describe provided functionality instead of only referring to
https://golang.org/pkg/sync.

golang/_sync.pxd

@@ -19,9 +19,11 @@
 # See https://www.nexedi.com/licensing for rationale and options.
 """Package sync mirrors Go package sync.
 
-See the following link about Go sync package:
+ - `Once` allows to execute an action only once.
+ - `WaitGroup` allows to wait for a collection of tasks to finish.
+ - `Sema` and `Mutex` provide low-level synchronization.
 
-    https://golang.org/pkg/sync
+See also https://golang.org/pkg/sync for Go sync package documentation.
 """
 
 cdef extern from "golang/libgolang.h" namespace "golang::sync" nogil:

golang/sync.py

@@ -17,11 +17,14 @@
 #
 # See COPYING file for full licensing terms.
 # See https://www.nexedi.com/licensing for rationale and options.
-"""Package sync mirrors Go package sync.
+"""Package sync mirrors and amends Go package sync.
 
-See the following link about Go sync package:
+ - `WorkGroup` allows to spawn group of goroutines working on a common task.
+ - `Once` allows to execute an action only once.
+ - `WaitGroup` allows to wait for a collection of tasks to finish.
+ - `Sema` and `Mutex` provide low-level synchronization.
 
-    https://golang.org/pkg/sync
+See also https://golang.org/pkg/sync for Go sync package documentation.
 """
 
 from __future__ import print_function, absolute_import