docs(plans): record the second round of plan-builder decisions

Decrypt through a plan is open, because StackCipher::decrypt already
exists with another signature. A plan has two starts and takes its
context from exactly one of three sources. The typed verb encrypt_into
and the picker are Rust-only. The derive emits the plan only after its
grammar is narrowed and the plan's widened, so the two stay one grammar.
EQL types are assembled per language from standard outputs, with no
registry and no target name in the data grammar.

The lowering table now names what #1068, #1069 and #1071 shipped.

Author: coderdan