Clarify documentation for sending mail via MUA like Thunderbird: do not specify a "sent" mail folder

[ajkessel]

A typical MUA configuration as with Thunderbird or mutt has the user specify a folder for saving sent mail. This won't work with Protonmail and Hydroxide because the "sent" mail folder is read-only. But Proton will automatically save any message sent through it to "sent", so there is no need for the end-user to configure any "sent" mail folder unless, for example, they want a redundant local copy.

The documentation could be clarified since otherwise users will always get an error when sending messages via Hydroxide and an end-user MUA like Thunderbird. Specifically, the documentation should recommend that the user not specify any folder to save sent mail too, since it will be automatically handed on the backend by Proton.