pw_allocator: Doxygenify Block::MergeNext() and Block::MergePrev()

Change-Id: I47548a790ac097bd358b288ec4c7dc377c1fec16
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/151251
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Kayce Basques <kayce@google.com>
Reviewed-by: Alexei Frolov <frolv@google.com>
Pigweed-Auto-Submit: Kayce Basques <kayce@google.com>
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
diff --git a/pw_allocator/public/pw_allocator/block.h b/pw_allocator/public/pw_allocator/block.h
index b63456a..cfe2c09 100644
--- a/pw_allocator/public/pw_allocator/block.h
+++ b/pw_allocator/public/pw_allocator/block.h
@@ -150,27 +150,29 @@
   //                       would not be large enough to store a block header.
   Status Split(size_t head_block_inner_size, Block** new_block);
 
-  // Merge this block with the one that comes after it.
-  // This function will not merge blocks if either are in use.
-  //
-  // This may return the following:
-  //   OK: Merge was successful.
-  //   OUT_OF_RANGE: Attempting to merge the "last" block.
-  //   FAILED_PRECONDITION: The blocks could not be merged because one of them
-  //                        was in use.
+  /// Merges this block with the one that comes after it.
+  ///
+  /// @pre The blocks must not be in use.
+  ///
+  /// @returns One of the following:
+  /// * `OK`: The merge was successful.
+  /// * `OUT_OF_RANGE`: Attempting to merge the last block failed.
+  /// * `FAILED_PRECONDITION`: The blocks could not be merged
+  ///   because one of them was in use.
   Status MergeNext();
 
-  // Merge this block with the one that comes before it.
-  // This function will not merge blocks if either are in use.
-  //
-  // Warning: merging with a previous block will invalidate this block instance.
-  // do not perform any operations on this instance after merging.
-  //
-  // This may return the following:
-  //   OK: Merge was successful.
-  //   OUT_OF_RANGE: Attempting to merge the "first" block.
-  //   FAILED_PRECONDITION: The blocks could not be merged because one of them
-  //                        was in use.
+  /// Merges this block with the one that comes before it.
+  ///
+  /// @pre The blocks must not be in use.
+  ///
+  /// @warning Merging with a previous block invalidates this block instance.
+  /// Do not perform any operations on this instance after merging.
+  ///
+  /// @returns One of the following:
+  /// * `OK`: The merge was successful.
+  /// * `OUT_OF_RANGE`: Attempting to merge the first block failed.
+  /// * `FAILED_PRECONDITION`: The blocks could not be merged because
+  ///   one of them was in use.
   Status MergePrev();
 
   // Returns whether this block is in-use or not