Refactor: Improve docstring for symtype

Author: bowenszhu

src/types.jl

@@ -100,9 +100,10 @@ end
 ###
 
 """
-    symtype(x)
+$(SIGNATURES)
 
-Returns the numeric type of `x`. By default this is just `typeof(x)`.
+Returns the [numeric type](https://docs.julialang.org/en/v1/base/numbers/#Standard-Numeric-Types) 
+of `x`. By default this is just `typeof(x)`.
 Define this for your symbolic types if you want [`SymbolicUtils.simplify`](@ref) to apply rules
 specific to numbers (such as commutativity of multiplication). Or such
 rules that may be implemented in the future.