Add docstring for the flyweight factory function

Author: bowenszhu

src/types.jl

@@ -341,6 +341,27 @@ end
 ### Constructors
 ###
 
+"""
+$(TYPEDSIGNATURES)
+
+Implements hash consing (flyweight design pattern) for `BasicSymbolic` objects.
+
+This function checks if an equivalent `BasicSymbolic` object already exists. It uses a 
+custom hash function (`hash2`) incorporating metadata and symtypes to search for existing 
+objects in a `WeakValueDict` (`wvd`).  Due to the possibility of hash collisions (where 
+different objects produce the same hash), a custom equality check (`isequal2`) which 
+includes metadata comparison, is used to confirm the equivalence of objects with matching 
+hashes. If an equivalent object is found, the existing object is returned; otherwise, the 
+input `s` is returned. This reduces memory usage, improves compilation time for runtime 
+code generation, and supports built-in common subexpression elimination, particularly when 
+working with symbolic objects with metadata.
+
+Using a `WeakValueDict` ensures that only weak references to `BasicSymbolic` objects are 
+stored, allowing objects that are no longer strongly referenced to be garbage collected. 
+Custom functions `hash2` and `isequal2` are used instead of `Base.hash` and `Base.isequal` 
+to accommodate metadata without disrupting existing tests reliant on the original behavior 
+of those functions.
+"""
 function BasicSymbolic(s::BasicSymbolic)::BasicSymbolic
     h = hash2(s)
     t = get!(wvd, h, s)