Add note elaborating on what the directionality of the resolved value means

Author: catamorphism

spec/formatting.md

@@ -938,6 +938,16 @@ The _Default Bidi Strategy_ is defined as follows:
    1. Let `fmt` be the formatted string representation of the _resolved value_ of `exp`.
    1. Let `dir` be the directionality of the _resolved value_ of `exp`,
       one of « `'LTR'`, `'RTL'`, `'unknown'` », with the same meanings as for `msgdir`.
+      > [!NOTE]
+      > _Resolved values_ need to track two different pieces of metadata about directionality:
+      > the "isolate" flag (see the next step for how that is used), which determines
+      > whether the formatted result needs to be isolated; and the directionality itself.
+      > Each _function handler_ can have its own means for determining the directionality
+      > of the _resolved value_ it returns.
+      > Alternately, an implementation could elide this tracking and instead determine
+      > the directionality from the locale.
+      > The directionality SHOULD NOT be determined by introspecting
+      > the character sequence in `fmt`.
    1. Let the boolean value `isolate` be
       True if the `u:dir` _option_ of the _resolved value_ of `exp` has a value other than `'inherit'`,
       or False otherwise.