[partition] Document that the default FS is used, also when erasing

src/modules/partition/Config.cpp

@@ -293,7 +293,8 @@ Config::fillConfigurationFSTypes(const QVariantMap& configurationMap)
     }
 
     Q_ASSERT( !m_eraseFsTypes.isEmpty() );
-    m_eraseFsTypeChoice = m_eraseFsTypes.first();
+    Q_ASSERT( m_eraseFsTypes.contains( fsRealName ) );
+    m_eraseFsTypeChoice = fsRealName;
     Q_EMIT eraseModeFilesystemChanged( m_eraseFsTypeChoice );
 }
 

src/modules/partition/partition.conf

@@ -144,6 +144,9 @@ defaultFileSystemType:  "ext4"
 # above), but it is also possible to give users a choice:
 # list suitable filesystems here. A drop-down is provided
 # to pick which is the filesystems will be used.
+#
+# The value *defaultFileSystemType* is added to this list (with a warning)
+# if not present; the default pick is the *defaultFileSystemType*.
 availableFileSystemTypes:  ["ext4", "btrfs", "f2fs"]
 
 # Show/hide LUKS related functionality in automated partitioning modes.