doc: clarify that the unsigned type is promoted to (JuliaLang#48973)

Clarify that the unsigned type is promoted to, if the types differ only
in signedness. This choice was explained by Karpinski in
JuliaLang#9292 (comment)
but wasn't fully clear in the documentation.

Author: robertfeldt

doc/src/manual/conversion-and-promotion.md

@@ -233,11 +233,11 @@ julia> promote(1 + 2im, 3//4)
 ```
 
 Floating-point values are promoted to the largest of the floating-point argument types. Integer
-values are promoted to the larger of either the native machine word size or the largest integer
-argument type. Mixtures of integers and floating-point values are promoted to a floating-point
-type big enough to hold all the values. Integers mixed with rationals are promoted to rationals.
-Rationals mixed with floats are promoted to floats. Complex values mixed with real values are
-promoted to the appropriate kind of complex value.
+values are promoted to the largest of the integer argument types. If the types are the same size
+but differ in signedness, the unsigned type is chosen. Mixtures of integers and floating-point
+values are promoted to a floating-point type big enough to hold all the values. Integers mixed
+with rationals are promoted to rationals. Rationals mixed with floats are promoted to floats.
+Complex values mixed with real values are promoted to the appropriate kind of complex value.
 
 That is really all there is to using promotions. The rest is just a matter of clever application,
 the most typical "clever" application being the definition of catch-all methods for numeric operations