[NFC] Add header documentation to the SBError::GetCString() to clarify ownwership of the returned string.

[clayborg]

Summary

Title says it all!

lldb/include/lldb/API/SBError.h

public:
   SBError();
 
   SBError(const lldb::SBError &rhs);
 
   ~SBError();
 
   const SBError &operator=(const lldb::SBError &rhs);
 
+  /// Get the error string as a NULL terminated UTF8 c-string.
+  ///
+  /// This SBError object owns the returned string and this object must be kept
+  /// around long enough to use the returned string.
   const char *GetCString() const;
 
   void Clear();
 
   bool Fail() const;
 
   bool Success() const;