[v6] upload_multiple hidden input is clear_photos, should probably be _clear_photos

[tabacitu]

I was working on the v6 docs and noticed this in the upload_multiple field docs:

NOTE: This field uses a clear_{fieldName} input to send the deleted files from the frontend to the backend. In case you are using $guarded add it there.
Eg: protected $guarded = ['id', 'clear_photos'];

I wasn't aware this needs to be done, and it's odd to me. Because that field will NEVER get saved to the database, so why unguard it? It's probably because if we don't... it will get stripped, right? So then what if we add an underscore before it? AFAIK:

• that's the convention Laravel uses for "helper" parameters (eg. _csrf)
• in the stripping function I think we already don't strip underscore parameters