Update repr docstring to hint what it stands for. (#56761)

- Change `repr` docstring from "Create a string from any
value ..." to "Create a string representation of any value ...".
- Document that it typically emits parseable Julia code

---------

Co-authored-by: Ian Butterworth <[email protected]>

base/strings/io.jl

@@ -250,7 +250,9 @@ print(io::IO, s::Union{String,SubString{String}}) = (write(io, s); nothing)
 """
     repr(x; context=nothing)
 
-Create a string from any value using the 2-argument `show(io, x)` function.
+Create a string representation of any value using the 2-argument `show(io, x)` function,
+which aims to produce a string that is parseable Julia code, where possible.
+i.e. `eval(Meta.parse(repr(x))) == x` should hold true.
 You should not add methods to `repr`; define a [`show`](@ref) method instead.
 
 The optional keyword argument `context` can be set to a `:key=>value` pair, a