doc: Warn about compatibility issues with CIFS and restic

On Linux CIFS (SMB) seems to be incompatible with the async preemption
implementation of Go 1.14. CIFS seems not to restart syscalls (open,
read, chmod, readdir, ...) as expected by Go, which sets SA_RESTART for
its signal handler to have syscalls restarted automatically. This leads
to Go passing up lots of EINTR return codes to restic.

See restic#2659 for a detailed explanation.

doc/030_preparing_a_new_repo.rst

@@ -54,6 +54,13 @@ command and enter the same password twice:
    Remembering your password is important! If you lose it, you won't be
    able to access data stored in the repository.
 
+.. warning::
+
+   On Linux, storing the backup repository on a CIFS (SMB) share is not
+   recommended due to compatibility issues. Either use another backend
+   or set the environment variable `GODEBUG` to `asyncpreemptoff=1`.
+   Refer to GitHub issue #2659 for further explanations.
+
 SFTP
 ****