[std] Some uses of "Equivalent to `expression`" violate specification style

The style guidelines say

For "Equivalent to", a plain expression should appear only if it is of type void; for value-returning functions, use return expression; (see below).

However, this rule is followed very inconsistently. For example, https://eel.is/c++draft/istream.assign#1 uses an equivalent to swap(rhs) in operator=, without return. If you want a more recent example, see yield_value in https://eel.is/c++draft/coro.generator.promise#4

I haven't checked every use to figure out how much is broken, but it doesn't look too bad overall.

Start with the referenced style guideline and compare the operator= and yield_value clauses linked in the issue. Search the draft for other uses of "Equivalent to" and check each against whether it returns a value. Done means the inconsistent wording is identified and corrected throughout the affected specification text.