docs: add comment to leaf case in verify subtree roots inclusion (#272)

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## Overview

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<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit


- **Documentation**
- Enhanced clarity of error handling in the verification process by
adding explanatory comments regarding invalid ranges.

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---------

Co-authored-by: Rootul P <rootulp@gmail.com>

proof.go

@@ -529,6 +529,9 @@ func (proof Proof) VerifySubtreeRootInclusion(nth *NmtHasher, subtreeRoots [][]b
 		}
 
 		if end-start == 1 {
+			// At this level, we reached a leaf, but we couldn't find any range corresponding
+			// to needed leaf [start, end).
+			// This means that the initial provided [start, end) range was invalid.
 			return nil, fmt.Errorf("the provided range [%d, %d) does not reference a valid inner node", proof.start, proof.end)
 		}