Filename length restrictions are not clearly documented

[NicoEichen]

if .txt name has more than 8 characters it won't work.

[per1234]

This is documented here:
https://www.arduino.cc/en/Reference/SD

It uses short 8.3 names for files.

But, considering this is a fairly common source of problems, I think that sentence could be made a little more clear. And there is no mention at all of this restriction on the SD.open() reference page.