[LangRef] clarify the meaning of noimplicitfloat

Adds some more text to the documentation for the noimplicitfloat
function attribute. Hopefully, this makes it clearer what
qualifies an implicit vs. explicit float, without becoming overly
long or going into target-specific details.

Reviewed By: rnk, craig.topper

Differential Revision: https://reviews.llvm.org/D104061