add some details to docstring of "if" (#37593)

Author: jw3126

base/docs/basedocs.jl

@@ -646,6 +646,13 @@ otherwise the condition expression `x > y` is evaluated, and if it is true, the
 corresponding block is evaluated; if neither expression is true, the `else` block is
 evaluated. The `elseif` and `else` blocks are optional, and as many `elseif` blocks as
 desired can be used.
+
+In contrast to some other languages conditions must be of type `Bool`. It does not
+suffice for conditions to be convertible to `Bool`.
+```jldoctest
+julia> if 1 end
+ERROR: TypeError: non-boolean (Int64) used in boolean context
+```
 """
 kw"if", kw"elseif", kw"else"