Add ref and doctest for only (JuliaLang#42491)

Co-authored-by: Fredrik Ekre <[email protected]>

base/iterators.jl

@@ -1342,13 +1342,32 @@ length(s::Stateful) = length(s.itr) - s.taken
 """
     only(x)
 
-Returns the one and only element of collection `x`, and throws an `ArgumentError` if the
+Return the one and only element of collection `x`, or throw an [`ArgumentError`](@ref) if the
 collection has zero or multiple elements.
 
 See also [`first`](@ref), [`last`](@ref).
 
 !!! compat "Julia 1.4"
     This method requires at least Julia 1.4.
+
+# Examples
+```jldoctest
+julia> only(["a"])
+"a"
+
+julia> only("a")
+'a': ASCII/Unicode U+0061 (category Ll: Letter, lowercase)
+
+julia> only(())
+ERROR: ArgumentError: Tuple contains 0 elements, must contain exactly 1 element
+Stacktrace:
+[...]
+
+julia> only(('a', 'b'))
+ERROR: ArgumentError: Tuple contains 2 elements, must contain exactly 1 element
+Stacktrace:
+[...]
+```
 """
 @propagate_inbounds function only(x)
     i = iterate(x)