Value auto for dir probably shouldn't say "no explicit directionality"

[aphillips]

dir member
https://w3c.github.io/manifest/#dir-member

"auto" (default)
No explicit directionality.

The value auto isn't exactly the same as "no explicit directionality". It means that the direction of strings with this value should be estimated using the first-strong part of the bidi algorithm. It is an explicit value. It also implies that the value should be bidi isolated when displayed. Perhaps say "Direction determined from content" instead?

Note that content which fools the "first-strong" detection in auto is common: an actual direction value such as rtl or ltr is preferred to just relying on auto.

Note that there was some previous discussion of this topic in #926