Document the fields of VersionNumber (#50179)

`VersionNumber`'s fields are currently not documented, and therefore,
private by default.
However, if people want to use `VersionNumber` for anything other than
the documented comparison operations, users will have to access the
fields.

base/version.jl

@@ -16,6 +16,13 @@ alphanumeric annotations.
 `VersionNumber` objects can be compared with all of the standard comparison
 operators (`==`, `<`, `<=`, etc.), with the result following semver rules.
 
+`VersionNumber` has the the following public fields:
+- `v.major::Integer`
+- `v.minor::Integer`
+- `v.patch::Integer`
+- `v.prerelease::Tuple{Vararg{Union{Integer, AbstractString}}}`
+- `v.build::Tuple{Vararg{Union{Integer, AbstractString}}}`
+
 See also [`@v_str`](@ref) to efficiently construct `VersionNumber` objects
 from semver-format literal strings, [`VERSION`](@ref) for the `VersionNumber`
 of Julia itself, and [Version Number Literals](@ref man-version-number-literals)

test/version.jl

@@ -219,6 +219,15 @@ for major=0:3, minor=0:3, patch=0:3
     end
 end
 
+# VersionNumber has the promised fields
+let v = v"4.2.1-1.x+a.9"
+    @test v.major isa Integer
+    @test v.minor isa Integer
+    @test v.patch isa Integer
+    @test v.prerelease isa Tuple{Vararg{Union{Integer, AbstractString}}}
+    @test v.build isa Tuple{Vararg{Union{Integer, AbstractString}}}
+end
+
 # julia_version.h version test
 @test VERSION.major == ccall(:jl_ver_major, Cint, ())
 @test VERSION.minor == ccall(:jl_ver_minor, Cint, ())