[cpp/en] String length content addition (#4888)

* Add string.length() and string.size() example line

Adds a line under the Strings section to add total string length by
calculating the sum of two strings which have been given on output above
with their sizes found out using string::length() and string::size()
functions, with appropriate comment on this line.

Signed-off-by: Harshit Gupta <hkg1412@gmail.com>

* Add cstring strlen() example

Includes <cstring> header to demonstrate length of a character array
string which is C-compliant by using strlen() function available from
this header supported in C++.
Declared string is initialized with char array size [10] to just fit the
string contents with '\0' terminator.

Signed-off-by: Harshit Gupta <hkg1412@gmail.com>

---------

Signed-off-by: Harshit Gupta <hkg1412@gmail.com>

c++.html.markdown

@@ -228,10 +228,19 @@ cout << myString + myOtherString; // "Hello World"
 
 cout << myString + " You"; // "Hello You"
 
+// C++ string length can be found from either string::length() or string::size()
+cout << myString.length() + myOtherString.size(); // Outputs 11 (= 5 + 6).
+
 // C++ strings are mutable.
 myString.append(" Dog");
 cout << myString; // "Hello Dog"
 
+// C++ can handle C-style strings with related functions using cstrings
+#include <cstring>
+
+char myOldString[10] = "Hello CPP";
+cout << myOldString;
+cout << "Length = " << strlen(myOldString); // Length = 9
 
 /////////////
 // References