Documentation audit for glimmer-modifier-manager feature flag

[tomdale]

We would like to "go" the glimmer-modifier-manager feature flag that landed in canary.

We would like to request a Learning Team audit of the API docs as implemented today, as well as thoughts on whether the guides should be updated to cover this functionality.

[locks]

As implemented today, the Guides do not cover this sort of documentation.
There's some efforts underway to explore how to diversify the kinds of documentations we officially provide.