Update stringObject.adoc

[robkwhitehouse]

The reference page shows that various initialization options are available as constructor parameters, but it does not show that existing String objects may also be assigned a new value, and it does not describe what types can be assigned to an existing String object.

[robkwhitehouse]

Also the documentation should clarify exactly what happens during assignment - i.e. is the donor array contents copied across, or does the recipient String object just receive a reference?

[per1234]

So that it will be clear to other reviewers, I will add a note that this is not a complete PR, but more of a general proposal or request for the addition of a String assignment operator page, which does not exist at this time:

• https://github.com/arduino/reference-en/tree/master/Language/Variables/Data%20Types/String/Operators
• https://www.arduino.cc/reference/en/language/variables/data-types/string/operators/assignment/