Document new "mailer_dsn" parameter

[pitilux]

Wallabag 2.6.1 introduced a new mailer_dsn parameter (and removed all existing mailer_* parameters). This is a suggestion to add the related documentation.

Few comments/qeustions:

• I kept the old parameters since I'm not sure how you want to proceed here. Keep them documented or remove them?
• The Symfony documentation specifies that some parts of the string should be urlencoded. Hence the note.
• I kept the description short. Should it be made longer with all details from the old parameters?

[nicosomb]

I kept the old parameters since I'm not sure how you want to proceed here. Keep them documented or remove them?

Keep them documented for now is good for me.