document compiler procs regarding & (#20257)

compiler/transf.nim

@@ -806,6 +806,8 @@ proc getMergeOp(n: PNode): PSym =
   else: discard
 
 proc flattenTreeAux(d, a: PNode, op: PSym) =
+  ## Optimizes away the `&` calls in the children nodes and
+  ## lifts the leaf nodes to the same level as `op2`.
   let op2 = getMergeOp(a)
   if op2 != nil and
       (op2.id == op.id or op.magic != mNone and op2.magic == op.magic):
@@ -898,6 +900,9 @@ proc transformExceptBranch(c: PTransf, n: PNode): PNode =
     result = transformSons(c, n)
 
 proc commonOptimizations*(g: ModuleGraph; idgen: IdGenerator; c: PSym, n: PNode): PNode =
+  ## Merges adjacent constant expressions of the children of the `&` call into
+  ## a single constant expression. It also inlines constant expressions which are not
+  ## complex.
   result = n
   for i in 0..<n.safeLen:
     result[i] = commonOptimizations(g, idgen, c, n[i])