Add note in pg_rewind documentation about read-only files

Add note in pg_rewind documentation about read-only files

When performing pg_rewind, the presence of a read-only file which is not
accessible for writes will cause a failure while processing.  This can
cause the control file of the target data folder to be truncated,
causing it to not be reusable with a successive run.

Also, when pg_rewind fails mid-flight, there is likely no way to be able
to recover the target data folder anyway, in which case a new base
backup is the best option.  A note is added in the documentation as
well about.

Reported-by: Christian H.
Author: Michael Paquier
Reviewed-by: Andrew Dunstan
Discussion: https://postgr.es/m/20180104200633.17004.16377%40wrigleys.postgresql.org