In concat() - Arduino Reference the syntax for concat is given as
string.concat(string, string2)
Actually, sometimes it would be nice to add two strings on one go. But this syntax is not supported by the compiler.
Error message: no matching function for call to 'String::concat(String&, String&)'
Therefore the documentation should be corrected.
I have reported this issue to the tracker: String concat reference page is incorrect · Issue #4244 · arduino/Arduino · GitHub and suggested wording for the updated documentation. If you have any input on how this should be written please let me know.
Fixed as suggested in the issue. Thank you!