Document that Boolean attributes should never be initialized to true if they should be configurable from markup

[AndrewJakubowicz]

Context

This is an area of confusion and we don't have clear documentation on lit.dev relating to this case.

Initializing a boolean attribute to true results in confusing behavior, as you cannot declaratively remove the attribute that has been defaulted to true from markup.

Proposal

Add a warning or note to https://lit.dev/docs/templates/expressions/#boolean-attribute-expressions stating that boolean attributes should never be default initialized to true.

References

This has been discussed in the past:

• Kevin's comment: Can't declare Boolean attributes with default of true? Polymer/polymer#1812 (comment)
• Link to polymer documentation that explains the same issue: https://polymer-library.polymer-project.org/2.0/docs/devguide/properties#configuring-boolean-properties

Out of scope of this issue, but maybe this can be caught by the analyzer.

[arthurevans]

This just came up in a chat conversation today, as well. I think it's really more apropos to the Reactive properties section, because it's really about the attribute API of a given component. We don't really talk much about API patterns at the moment.