Add note in docs about incompatibility of backup types

Adds a note to both file-level and block-level incremental backup
sections to highlight that they're currently incompatible with each
other.

References: BAR-281

Signed-off-by: Gustavo William <gustavo.oliveira@enterprisedb.com>

doc/manual/50-feature-details.en.md

@@ -83,6 +83,10 @@ barman backup --reuse-backup=link <server_name>
 > **IMPORTANT:** The `reuse_backup` option must be used along with
 > `rsync` or `local-rsync` as backup method.
 
+> **NOTE:** Incremental backups of different types are currently incompatible i.e.
+> a file-level incremental backup can not be taken upon a block-level backup and
+> vice-versa.
+
 
 ### Block-level incremental backups
 
@@ -118,6 +122,10 @@ To be able to perform a block-level incremental backup in Barman you must:
 > it is adivised to take a new full-backup as divergent checkum configurations can potentially
 > lead to issues during recovery.
 
+> **NOTE:** Incremental backups of different types are currently incompatible i.e.
+> a block-level incremental backup can not be taken upon a file-level backup and
+> vice-versa.
+
 ### Limiting bandwidth usage
 
 It is possible to limit the usage of I/O bandwidth through the