Adding various docs to clarify structs / traits (solana-labs#140)

* Adding various docs to clarify structs / traits

Intended to be viewed via `docs.rs`. Locally, run `cargo doc --open` to preview

* Addressing comments

lib/concurrent-merkle-tree/src/error.rs

@@ -2,20 +2,35 @@ use thiserror::Error;
 
 #[derive(Error, Debug)]
 pub enum CMTError {
+    /// Received an index larger than the rightmost index
     #[error("Received an index larger than the rightmost index")]
     LeafIndexOutOfBounds,
+
+    /// Invalid root recomputed from proof
     #[error("Invalid root recomputed from proof")]
     InvalidProof,
+
+    /// Node to append cannot be empty
     #[error("Cannot append an empty node")]
     CannotAppendEmptyNode,
+
+    /// The tree is at capacity
     #[error("Tree is full, cannot append")]
     TreeFull,
+
+    /// This tree has already been initialized
     #[error("Tree already initialized")]
     TreeAlreadyInitialized,
+
+    /// Invalid number of bytes passed for node (expected 32 bytes)
     #[error("Invalid number of bytes passed for node (expected 32 bytes)")]
     InvalidNodeByteLength,
+
+    /// Fast forward error: we cannot find a valid point to fast-forward the current proof from
     #[error("Root not found in changelog buffer")]
     RootNotFound,
+
+    /// Valid proof was passed to a leaf, but it's value has changed since the proof was issued
     #[error(
         "Valid proof was passed to a leaf, but it's value has changed since the proof was issued"
     )]