[FEEDBACK] bidi clarifications

[macchiati]

We had a discussion around the implications of bidi isolation, in formatting.md. I'm capturing some items here for post 46.1

1. General issue. While it is clear that Default Bidi Strategy is the correct strategy in the general case, the algorithm seems to preclude various optimizations. It should make it clear that an implementation is conformant to the DBS if the bidi ordering it produces for any formatted pattern is identical to the bidi ordering produced for that pattern by the DBS.
2. It should also make clear than an implementation may generate equivalent results for other environments. Eg, when generating HTML, it could produce bidi markdown instead of the Unicode bidi control characters. This might be covered by #formatting, but best to have it specifically called out.
3. Details
   1. In #handling-bidirectional-text we have "Let msgdir be the directionality of the whole message," but it is not defined.
   2. In #formatting-context we have "Information on the base directionality of the message and its text tokens. This will be used by strategies for bidirectional isolation, and can be used to set the base direction of the message upon display." That sounds like it is supposed to define msgdir — or at least the connection between them needs to be clear.
   3. "Let fmt be the formatted string representation of the resolved value of exp." should be "Let fmt be the formatted string representation of exp."

A related issue: in #formatting it should make it clear that callers of implementations cannot rely on the literal text in a pattern being preserved in the formatted pattern. That is, an implementation could change the literal text, such as improving the result of {{You have an {$item} in your basket.}} based on the value of $item, eg "You have an apple in your basket." vs "You have a pear in your basket.".