Fix incorrect alias checking in docs--signature

help-split-fundoc expects the function symbol to be the function
name used in the docstring. Since we only use help-split-fundoc for
subrs, this isn't an issue in practice, because subrs use
`(fn some-arg some-other-arg)` rather than an explicit name.

Still, the boolean ought to do what it says it's doing.

(Ahem, sorry about this. Realised the code didn't do what I wanted it when I had another quick skim.)