libgolang: Provide top-level overview for defer

defer is now part of libgolang API (see 1d153a45 "libgolang: Expose
defer as public C++ API"). It should be explained in top-level overview.

golang/libgolang.h

@@ -38,6 +38,7 @@
 //  - `go` spawns new task.
 //  - `chan<T>`, and `select` provide channels with Go semantic and automatic
 //    lifetime management.
+//  - `defer` schedules cleanup.
 //  - `panic` throws exception that represent C-level panic.
 //
 // For example:
@@ -59,6 +60,10 @@
 //      if (_ == 2)
 //          // case 2 selected: j received from ch
 //
+//      defer([]() {
+//          printf("leaving...\n");
+//      });
+//
 //      if (<bug condition>)
 //          panic("bug");
 //