[docs] suggest fix over a new constraint (#2645)

docs/src/manual/variables.md

@@ -239,6 +239,7 @@ julia> unfix(x)
 julia> is_fixed(x)
 false
 ```
+
 Fixing a variable with existing bounds will throw an error. To delete the bounds
 prior to fixing, use `fix(variable, value; force = true)`.
 
@@ -256,6 +257,10 @@ julia> fix_value(x)
 2.0
 ```
 
+!!! tip
+    Use [`fix`](@ref) instead of `@constraint(model, x == 2)`. The former modifies
+    variable bounds, while the latter adds a new linear constraint to the problem.
+
 ## Variable names
 
 The name, i.e. the value of the `MOI.VariableName` attribute, of a variable can