Remove doc comments from concrete floating-point types (#26399)

* Remove doc comments from concrete floating-point types

... when they are already present on the protocol. I don't *think* that we need them anymore for xcode documentation purposes. There are reasonable arguments both ways on this:
1. when you're editing the concrete implementations, it's sometimes nice to have the doc comment right there.
2. but it needlessly repetitive, and introduces the opportunity for comments to get out of sync.
3. it also adds noise; it would be nice for information density if the implementation only had implementation notes.

* Add examples to documentation for FloatingPoint.isCanonical

In particular, document that subnormal encodings are treated as non-canonical zeros on platforms that flush to zero.

* Additional fixes by @xwu and @natecook1000 .

Author: stephentyrone