Add examples explaining initialization heuristics

Adds documentation explaining that variables are considered initialized
after being passed by pointer or reference to another function, thereby
assuming the callee will initialize the variable. Since this may not
always be desired, we also suggest using SAL annotations to more
precisely describe the function behavior.

Author: eholk

docs/code-quality/c6001.md

@@ -37,7 +37,7 @@ To correct this warning, initialize the variable as shown in the following code:
 ```cpp
 int f( bool b )
 {
-   int i= -1;
+   int i = -1;
 
    if ( b )
    {
@@ -47,6 +47,57 @@ int f( bool b )
 }
 ```
 
+## Heuristics
+
+Variables are also considered initialized when they are passed by reference to
+another function. Thus, the example below would also consider `i` to be initialized.
+
+```cpp
+void init( int& i );
+
+int f( bool b )
+{
+   int i;
+
+   init(i);
+
+   if ( b )
+   {
+      i = 0;
+   }
+   return i; // i is assumed to be initialized by init(i)
+}
+```
+
+This is to support the pattern of passing a pointer to a variable into an
+initialization function.
+
+Note that many functions expect pointers to point to initialized data already,
+in which case this heuristic would lead to false negatives. [SAL annotations]
+such as `_In_` and `_Out_` can be used to more precisely describe a function's
+behavior. For example, in the following we call an external function that
+expects its argument to already be initialized and the warning is still
+generated.
+
+```cpp
+void use( _In_ int& i );
+
+int f( bool b )
+{
+   int i;
+
+   use(i); // use assumes its argument is already initialized due to the _In_ annotation
+
+   if ( b )
+   {
+      i = 0;
+   }
+   return i; // i is uninitialized if b is false
+}
+```
+
+[SAL annotations]: ./annotating-function-parameters-and-return-values.md
+
 ## See also
 
 [Compiler Warning (level 1 and level 4) C4700](../error-messages/compiler-warnings/compiler-warning-level-1-and-level-4-c4700.md)