const fn shown in docs when not available

[jethrogb]

Regression from 1.26.0 to 1.27.0 and later. See for example
https://doc.rust-lang.org/1.26.0/std/vec/struct.Vec.html#method.new
https://doc.rust-lang.org/1.27.0/std/vec/struct.Vec.html#method.new
See also #31329 and #31098