Improve docstrings

[samestep]

Currently if you hover over Real in this example in VS Code:

import { Real } from "rose";

The following docstring appears:

An abstract 64-bit floating point number. The 64-bit floating-point type.

This is the concatenation of the docstrings for the static and dynamic meanings of Real:

rose/packages/core/src/impl.ts

Lines 65 to 66 in 603760a

	/** An abstract 64-bit floating point number. */
	export type Real = number | Var;

rose/packages/core/src/impl.ts

Lines 215 to 216 in 603760a

	/** The 64-bit floating-point type. */
	export const Real = Symbol("Real");

We should fix this so that the concatenated docstring makes more sense.

Also, many of the docstrings are too terse or simply wrong; for instance, the interp docstring says it only works on "nullary" functions, which is incorrect as of #80:

rose/packages/core/src/impl.ts

Lines 555 to 556 in 603760a

	/** Concretize the nullary abstract function `f` using the interpreter. */
	export const interp =