Suggest the use of `assume_spherical_screen()` in the autoalign/reproject examples

[ayshih]

These two examples:

• Autoaligning: https://docs.sunpy.org/en/v3.1.5/generated/gallery/map_transformations/autoalign_aia_hmi.html
• Reprojecting: https://docs.sunpy.org/en/v3.1.5/generated/gallery/map_transformations/reprojection_align_aia_hmi.html

are silent about the fact that off-disk HMI data is being discarded. The examples should note that, and additionally suggest that the user can use Helioprojective.assume_spherical_screen() to retain the off-disk HMI data.