ifDefined documentation updates for Lit 2

[AndrewJakubowicz]

Note: TODO - The behavior is much more subtle. It's an initial render that creates the behavior difference. After that ifDefined is indeed a noop. See this repro: https://lit.dev/playground/?mods=gists#gist=7ecdc1acf63b0f249e6e582fb8a526aa

Context

The documentation currently states that using ifDefined on something that isn't an attribute part is a no-op, however at runtime ifDefined correctly shows the property default, whilst undefined doesn't. This contradicts that it is a no-op.

There was a change in behavior between Lit 1 and Lit 2 with the ifDefined directive, where some previously no-op behavior is now valid usage. See runem/lit-analyzer#223 for more details.

There are 3 places where ifDefined needs to be updated:

1. In the Lit Analyzer repo: Confusing ifDefined extension error when author wants default property fallback behavior runem/lit-analyzer#223
2. Generated JSDoc (related to [all] Expand jsdoc documentation lit#1883 (comment))
3. Website directive docs
   1. Directives list
   2. Built in Directives

Completion criteria

• Update lit.dev documentation. Other places to update are mentioned for completeness.

[AndrewJakubowicz]

Closing this issue as ifDefined is a red herring. The tricky nuance is in how nothing behaves in reactive property updates.

It's an initial render that creates the behavior difference. After that ifDefined is indeed a noop. See this repro: https://lit.dev/playground/?mods=gists#gist=7ecdc1acf63b0f249e6e582fb8a526aa