Update main functions with omitted int return type

Author: Rageking8

docs/error-messages/compiler-errors-1/compiler-error-c2086.md

@@ -18,7 +18,7 @@ The following sample generates C2086:
 
 ```cpp
 // C2086.cpp
-main() {
+int main() {
   int a;
   int a;   // C2086 not an error in ANSI C
 }

docs/error-messages/compiler-warnings/compiler-warning-level-1-c4744.md

@@ -37,7 +37,7 @@ The following sample generates C4744.
 
 extern unsigned global;
 
-main()
+int main()
 {
     printf_s("%d\n", global);
 }

docs/error-messages/tool-errors/math-error-m6111.md

@@ -14,9 +14,9 @@ A floating-point operation resulted in a stack underflow on the 8087/287/387 cop
 
 This error is often caused by a call to a **`long double`** function that does not return a value. For example, the following generates this error when compiled and run:
 
-```
+```c
 long double ld() {};
-main ()
+int main ()
 {
   ld();
 }

docs/intrinsics/debugbreak.md

@@ -34,15 +34,15 @@ The `__debugbreak` compiler intrinsic, similar to [DebugBreak](/windows/win32/ap
 For example:
 
 ```C
-main() {
+int main() {
    __debugbreak();
 }
 ```
 
 is similar to:
 
 ```C
-main() {
+int main() {
    __asm {
       int 3
    }

docs/parallel/openmp/a-examples.md

@@ -854,7 +854,7 @@ The following example demonstrates the [num_threads](2-directives.md#23-parallel
 
 ```cpp
 #include <omp.h>
-main()
+int main()
 {
     omp_set_dynamic(1);
     ...