Added a new Reserved Field Names section to the migration guide.
Clarified that certain field names (`__v`, `salt`, `hash`, `file`, etc.)
are reserved for internal use and will be sanitized from the config if
used.
Included additional reserved names specific to `MongoDB`, `auth`-enabled
collections, and `upload`-enabled collections.
Added a note recommending against using field names with an underscore
(`_`) prefix, as they are reserved for internal columns and may cause
conflicts in `SQL` and other contexts.
Fixes#11159
bcc68572bf