Add explicit default "inherit" value

Author: eemeli

spec/formatting.md

@@ -939,8 +939,8 @@ The _Default Bidi Strategy_ is defined as follows:
    1. Let `dir` be the directionality of `fmt`,
       one of « `'LTR'`, `'RTL'`, `'unknown'` », with the same meanings as for `msgdir`.
    1. Let the boolean value `isolate` be
-      True if the `u:dir` _option_ is set on the _resolved value_ of `exp`,
-      or False if it is not set or the implementation does not support the option.
+      True if the `u:dir` _option_ of the _resolved value_ of `exp` has a value other than `'inherit'`,
+      or False otherwise.
    1. If `dir` is `'LTR'`:
       1. If `msgdir` is `'LTR'` in the formatted output
          and `isolate` is False,

spec/u-namespace.md

@@ -84,6 +84,8 @@ or a _variable_ whose _resolved value_ is one of these _literals_:
 - `ltr`: left-to-right directionality
 - `rtl`: right-to-left directionality
 - `auto`: directionality determined from _expression_ contents
+- `inherit` (default): directionality inherited from the _message_,
+  without isolation of the _expression_ value
 
 For other values, a _Bad Option_ error is emitted
 and the value of the `u:dir` option is ignored.