docs: add note about not implying being defined to names

ref #51584

Author: nsajko

base/reflection.jl

@@ -84,6 +84,12 @@ are also included.
 As a special case, all names defined in `Main` are considered \"public\",
 since it is not idiomatic to explicitly mark names from `Main` as public.
 
+!!! note
+    `sym ∈ names(SomeModule)` does *not* imply `isdefined(SomeModule, sym)`,
+    because the value returned by `names` will contain a symbol marked with `public`
+    (or `export`) in the module, even if that symbol does not correspond to any names
+    defined in the module.
+
 See also: [`isexported`](@ref), [`ispublic`](@ref), [`@locals`](@ref Base.@locals), [`@__MODULE__`](@ref).
 """
 names(m::Module; all::Bool = false, imported::Bool = false) =