Add a note about mail headers overwriting

Since some headers can appear multiple times in an email it's required
to set it to nil first when you want to overwrite an existing one.

This commit add some information about this process.

Fix #15912

[ci skip]
What about something like : 
```you need to set it to nil first to reset it. ```
because this seem like Mail is a self-conscious being :)
`so that Mail knows you are`