Improve documentation of unary : (#45426)

Unary `:` was poorly documented previously. The previous docs stated it would
return a Symbol, but in fact also Expr objects and literal values can be
returned from code quoting.

See #43054

Co-authored-by: Shuhei Kadowaki <[email protected]>
Co-authored-by: Valentin Churavy <[email protected]>

base/docs/basedocs.jl

@@ -682,6 +682,33 @@ Expr
 """
 Expr
 
+"""
+ (:)(expr)
+
+`:expr` quotes the expression `expr`, returning the abstract syntax tree (AST) of `expr`.
+The AST may be of type `Expr`, `Symbol`, or a literal value.
+Which of these three types are returned for any given expression is an
+implementation detail.
+
+See also: [`Expr`](@ref), [`Symbol`](@ref), [`Meta.parse`](@ref)
+
+# Examples
+```jldoctest
+julia> expr = :(a = b + 2*x)
+:(a = b + 2x)
+
+julia> sym = :some_identifier
+:some_identifier
+
+julia> value = :0xff
+0xff
+
+julia> typeof((expr, sym, value))
+Tuple{Expr, Symbol, UInt8}
+```
+"""
+(:)
+
 """
  \$
 

base/range.jl

@@ -34,8 +34,7 @@ _colon(::Any, ::Any, start::T, step, stop::T) where {T} =
 Range operator. `a:b` constructs a range from `a` to `b` with a step size of 1 (a [`UnitRange`](@ref))
 , and `a:s:b` is similar but uses a step size of `s` (a [`StepRange`](@ref)).
 
-`:` is also used in indexing to select whole dimensions
- and for [`Symbol`](@ref) literals, as in e.g. `:hello`.
+`:` is also used in indexing to select whole dimensions, e.g. in `A[:, 1]`.
 """
 (:)(start::T, step, stop::T) where {T} = _colon(start, step, stop)
 (:)(start::T, step, stop::T) where {T<:Real} = _colon(start, step, stop)