[Documentation] Add clarifying docs for ClosestProof (#32)

docs/SMT.md

@@ -4,26 +4,26 @@
 
 - [Overview](#overview)
 - [Implementation](#implementation)
-  - [Inner Nodes](#inner-nodes)
-  - [Extension Nodes](#extension-nodes)
-  - [Leaf Nodes](#leaf-nodes)
-  - [Lazy Nodes](#lazy-nodes)
-  - [Lazy Loading](#lazy-loading)
-  - [Visualisations](#visualisations)
-    - [General Trie Structure](#general-trie-structure)
-    - [Lazy Nodes](#lazy-nodes-1)
+  * [Inner Nodes](#inner-nodes)
+  * [Extension Nodes](#extension-nodes)
+  * [Leaf Nodes](#leaf-nodes)
+  * [Lazy Nodes](#lazy-nodes)
+  * [Lazy Loading](#lazy-loading)
+  * [Visualisations](#visualisations)
+    + [General Trie Structure](#general-trie-structure)
+    + [Lazy Nodes](#lazy-nodes-1)
 - [Paths](#paths)
-  - [Visualisation](#visualisation)
+  * [Visualisation](#visualisation)
 - [Values](#values)
-  - [Nil values](#nil-values)
+  * [Nil values](#nil-values)
 - [Hashers & Digests](#hashers--digests)
 - [Proofs](#proofs)
-  - [Verification](#verification)
-  - [Closest Proof](#closest-proof)
-  - [Compression](#compression)
-  - [Serialisation](#serialisation)
+  * [Verification](#verification)
+  * [Closest Proof](#closest-proof)
+  * [Compression](#compression)
+  * [Serialisation](#serialisation)
 - [Database](#database)
-  - [Data Loss](#data-loss)
+  * [Data Loss](#data-loss)
 - [Sparse Merkle Sum Trie](#sparse-merkle-sum-trie)
 - [Example](#example)
 
@@ -378,10 +378,28 @@ This backstepping process allows the traversal to continue until it reaches a
 sentinel leaf that has the longest common prefix and most bits in common with
 the provided hash, up to the depth of the leaf found.
 
-This method guarentees a proof of inclusion in all cases and can be verified by
+This method guarantees a proof of inclusion in all cases and can be verified by
 using the `VerifyClosestProof` function which requires the proof and root hash
 of the trie.
 
+Since the `ClosestProof` method takes a hash as input, it is possible to place a
+leaf in the trie according to the hash's path, if it is known. Depending on
+the use case of this function this may expose a vulnerability. _It is not
+intendend to be used as a general purpose proof mechanism_. Given two parties:
+the prover and the verifier, the verifier should supply the prover with a hash
+after the trie has been "closed" and is no longer being updated. The prover
+will then generate a `ClosestProof` for a leaf using the corresponding method.
+The verifier can subsequently verify that proof for its validity. If however,
+the prover were to know the hash prior to "closing" the trie, they could place
+a leaf where the method would always guarantee it existence. This form of attack
+can only happen due to the method's deterministic behaviour and the prover
+knowing the hash before they have "closed" the trie. The intended use of this
+method is that the verifier gives the hash only after the prover has closed their
+trie and submitted the closed trie's root hash. This enables the verifier to
+verify the integrity of the proof (if the trie was changed the root hash would
+be different) and also guarantees the pseudo-random proof of inclusion was not
+a maliciously placed leaf.
+
 ### Compression
 
 Both proof types have compression and decompression functions available to