perlfunc: Improve the pointer from "elseif" to "elsif"

A co-worker pointed out that the docs for "elsif" were quite confusing
because nothing when you "perldoc -f elseif" points out that it doesn't
exist, it just directs you to perlsyn where we only document "elsif".

Ricardo Signes added this aliasing back in v5.15.7-194-g8f0d6a6.

Improve this confusion, and also add a mention of the common "elif" and
"else if" variants while I'm at it. I was originally going to just alias
them, but t/porting/perlfunc.t started failing because we're missing
cross-references, and unlike "elseif" the other two aren't keywords,
even if the "elseif" one is only here to warn you about its use.