Add comment directive to manually specify call expr

[bkryza]

In some cases, e.g when creating threads or some custom thread-like objects (e.g. fibers) it's not easy to infer from the code that another function or method is called at this code position.

There should be a custom comment directive, allowing to manually insert a call expression in the comment, saying that the next line of code invokes a specific function or method, e.g.:

void a(int arg) { }

void b() {
    // \uml{call a(int)}
    boost::fibers::fiber([&]() {
        a(0);
    }).detach();
}