Improve `identity` documentation
Use more specific word than “stream” in bridging
Replace single-char type names where confusing

@robrix @mdiep There were a lot of places I neglected to update, simply
because T (and T -> U, etc.) is often clearer than a meaningless word
like “Value” or “Element.”

So, this change affects the spots where the single character was
actually an abbreviation for something meaningful, but doesn’t go
further than that.
Refactor `try` in terms of `tryMap`
Flip scheduler disposal check