fix #16678, add a note about when : symbol quoting syntax works (#32436)

Author: JeffBezanson

doc/src/manual/metaprogramming.md

@@ -126,6 +126,9 @@ julia> Symbol(:var,'_',"sym")
 :var_sym
 ```
 
+Note that to use `:` syntax, the symbol's name must be a valid identifier.
+Otherwise the `Symbol(str)` constructor must be used.
+
 In the context of an expression, symbols are used to indicate access to variables; when an expression
 is evaluated, a symbol is replaced with the value bound to that symbol in the appropriate [scope](@ref scope-of-variables).