Docs: astro-images: Add help notice on case sensitivity of glob

[tordans]

I hope this will help with debugging something that is tedious to debug when you don't expect this to be a possible error.

I tried improving the Astro docs at the source first in https://github.com/withastro/docs/pull/10169/files but it looks like the case-sensitivity is something that Astro devs are supposed to know about (I did not…), so that was rejected.