[analyzer][docs] Ellaborate the docs of cplusplus.StringChecker

[steakhal]

Summary

Let's describe accurately what the users can expect from the checker in
a direct way.
Also, add an example warning message.

[Szelethus]

clang/docs/analyzer/checkers.rst

322

You are not wrong, this just comes across too negative, and, pardon me, but pointlessly negative. I'd just spin this as "The current implementation only warns if an std::string was constructed from a null pointer".

324

327

I'd delete this as well. This should be implied by the documentation of the checker.

[steakhal]

clang/docs/analyzer/checkers.rst

322

Hm, right.

[Szelethus]

Maybe we should change the name of this checker to NullConstructedString or something, instead of piling even more different kinds of warnings into it in the future. Or maybe not. In any case, the patch LGTM!

[steakhal]

Add reference to the corresponding CERT rule.

[martong]

LGTM!

clang/docs/analyzer/checkers.rst

 Checks C++ copy and move assignment operators for self assignment.
 
 .. _cplusplus-StringChecker:
 
 cplusplus.StringChecker (C++)
 """""""""""""""""""""""""""""
 Checks std::string operations.
 
+Checks if the cstring pointer from which the ``std::string`` object is
+constructed is ``NULL`` or not.
+If the checker cannot reason about the nullness of the pointer it will assume
+that it was non-null to satisfy the precondition of the constructor.
+
+This checker is capable of checking the `SEI CERT C++ coding rule STR51-CPP.
+Do not attempt to create a std::string from a null pointer
+<https://wiki.sei.cmu.edu/confluence/x/E3s-BQ>`__.
+
 .. code-block:: cpp
 
  #include <string>
 
  void f(const char *p) {
    if (!p) {
-     std::string msg(p); // warn: p is NULL
+     std::string msg(p); // warn: The parameter must not be null
    }
  }
 
 .. _deadcode-checkers:
 
 deadcode
 ^^^^^^^^