MAVFTP URL clarifications

[hamishwillee]

This updates the mavftp URL section to match the proposal from the 20260603 -Dev-Meeting, as outline by @julianoes in mavlink/mavlink#2484 (comment)

@peterbarker This is the following, where you can't have an absolute path with @SOMEDIR:

mftp:///absolute/path
mftp://relative/path

mftp://comp=12:/absolute/path
mftp://comp=12:relative/path

mftp://@SOMEDIR/relative/path
mftp://comp=12:@SOMEDIR/relative/path

Fixes mavlink/mavlink#2484

[hamishwillee]

@peterbarker Any comments. I plan to merge this next time I'm around.

[hamishwillee]

OK, everyone has had a chance to look at this. Thanks @julianoes