add parenthetical to manual regarding absence of Float typealias

Seems to be a common FAQ. Not sure if those users would have seen it here,
but it's right after explaining the existance of the `Int` type-alias.

Author: vtjnash

doc/src/manual/types.md

@@ -1064,6 +1064,10 @@ end
 Of course, this depends on what `Int` is aliased to -- but that is predefined to be the correct
 type -- either `Int32` or `Int64`.
 
+(Note that unlike `Int`, `Float` does not exist as a type-alias for a specific sized `AbstractFloat`.
+Unlike with integer registers, the floating point register sizes are specified by the IEEE-754 standard.
+Whereas the size of `Int` reflects the size of a native pointer on that machine.)
+
 For parametric types, `typealias` can be convenient for providing names for cases where some of
 the parameter choices are fixed: