document kwarg fold in substitute

Author: benneti

src/Symbolics.jl

@@ -64,9 +64,10 @@ include("rewrite-helpers.jl")
 include("complex.jl")
 
 """
-    substitute(expr, s)
+    substitute(expr, s; fold=true)
 
 Performs the substitution on `expr` according to rule(s) `s`.
+If `fold=false`, expressions which can be evaluated won't be evaluated.
 # Examples
 ```jldoctest
 julia> @variables t x y z(t)
@@ -79,6 +80,8 @@ julia> ex = x + y + sin(z)
 (x + y) + sin(z(t))
 julia> substitute(ex, Dict([x => z, sin(z) => z^2]))
 (z(t) + y) + (z(t) ^ 2)
+julia> substitute(sqrt(2x), Dict([x => 1]); fold=false)
+sqrt(2)
 ```
 """
 substitute