[cxx-string-captured] Improve the documentation

Summary: We updated the checker in a previous diff but the documentation was still the old one.

Reviewed By: skcho

Differential Revision: D65140447

fbshipit-source-id: 30f2939b8115ec2a987f53751410c3cd6607f608

infer/documentation/issues/CXX_STRING_CAPTURED_IN_BLOCK.md

@@ -1,19 +1,15 @@
-This check flags when a local variable of type `std::string` is captured in an escaping block.
+This check flags when an internal pointer of a local variable of type `std::string` is captured in an escaping block.
 This means that the block will be leaving the current scope, i.e. it is
 not annotated with `__attribute__((noescape))`.
 
 Example:
 
 ```
-- (void)string_captured_in_escaping_block_bad {
   std::string fullName;
+  const char* c = fullName.c_str();
   dispatch_async(dispatch_get_main_queue(), ^{
-    const char* c = fullName.c_str();
-    ...
+    const char* c1 = c;
   });
-  ...;
-}
 ```
 
-This could cause crashes because the variable is likely to be freed if the block
-uses it later.
+This could cause crashes because the variable is likely to be freed when the code is executed, leaving the pointer dangling.