Merge #15659: [docs] fix findFork comment

c968780 [docs] fix comment: the return value of findFork is _not_ an ancestor when the specified block is on the active chain (r8921039)

Pull request description:

  The return value of findFork is an ancestor of the specified block only when specified block is _not_ on the active chain. When it is on the active chain, the return value is the specified block itself, not an ancestor of it.

ACKs for commit c96878:
  promag:
    utACK c968780, however comment could be shorter.
  ryanofsky:
    utACK c968780. Only change since last review is squash

Tree-SHA512: bb05d734059898784c4a59b5b0344719eb4dfb2d49a0f7f705fcb2eb630702e66be81c01299185faf0c219fa9f9aa64cbdf6d5f91e0b3dce0ff420909a454a18

Author: MarcoFalke

src/interfaces/chain.h

@@ -120,8 +120,9 @@ class Chain
         //! nullopt if no block in the range is pruned. Range is inclusive.
         virtual Optional<int> findPruned(int start_height = 0, Optional<int> stop_height = nullopt) = 0;
 
-        //! Return height of the highest block on the chain that is an ancestor
-        //! of the specified block, or nullopt if no common ancestor is found.
+        //! Return height of the specified block if it is on the chain, otherwise
+        //! return the height of the highest block on chain that's an ancestor
+        //! of the specified block, or nullopt if there is no common ancestor.
         //! Also return the height of the specified block as an optional output
         //! parameter (to avoid the cost of a second hash lookup in case this
         //! information is desired).
@@ -135,9 +136,9 @@ class Chain
         //! Get locator for the current chain tip.
         virtual CBlockLocator getTipLocator() = 0;
 
-        //! Return height of the latest block common to locator and chain, which
-        //! is guaranteed to be an ancestor of the block used to create the
-        //! locator.
+        //! Return height of the highest block on chain in common with the locator,
+        //! which will either be the original block used to create the locator,
+        //! or one of its ancestors.
         virtual Optional<int> findLocatorFork(const CBlockLocator& locator) = 0;
 
         //! Check if transaction will be final given chain height current time.