Add doc about incremental backup recover in the manual

[andremagui]

There are changes in the behavior of barman recover with the new incremental backup feature. This PR will add the proper documentation about the recover of incremental backups.

This needs to address:

1. Add note with a brief description of how Barman recovers the backups, mainly mentioning the use of pg_combinebackup, etc.
2. Mention the use of the local_staging_path to place the pg_combinebackup synthetic backup.
3. Add note, with links to the PG docs about checksums being on and off in different backups of a chain of backups.
4. Add field and details about local_staging_path to args in the man1 recover command documentation.

References: BAR-252

[andremagui]

We still need to discuss about the recovery_staging_path topic posted in the barman channel, so i can modify the documentation related to this.

[barthisrael]

Let's squash before merging.