Document that merge strategies can now take their own options

Also document the recently added -Xtheirs, -Xours and -Xsubtree[=path]
options to the merge-recursive strategy.

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/merge-options.txt

@@ -74,3 +74,8 @@ option can be used to override --squash.
 -v::
 --verbose::
 	Be verbose.
+
+-X <option>::
+--strategy-option=<option>::
+	Pass merge strategy specific option through to the merge
+	strategy.

Documentation/merge-strategies.txt

@@ -1,6 +1,11 @@
 MERGE STRATEGIES
 ----------------
 
+The merge mechanism ('git-merge' and 'git-pull' commands) allows the
+backend 'merge strategies' to be chosen with `-s` option.  Some strategies
+can also take their own options, which can be passed by giving `-X<option>`
+arguments to 'git-merge' and/or 'git-pull'.
+
 resolve::
 	This can only resolve two heads (i.e. the current branch
 	and another branch you pulled from) using a 3-way merge
@@ -20,6 +25,27 @@ recursive::
 	Additionally this can detect and handle merges involving
 	renames.  This is the default merge strategy when
 	pulling or merging one branch.
++
+The 'recursive' strategy can take the following options:
+
+ours;;
+	This option forces conflicting hunks to be auto-resolved cleanly by
+	favoring 'our' version.  Changes from the other tree that do not
+	conflict with our side are reflected to the merge result.
++
+This should not be confused with the 'ours' merge strategy, which does not
+even look at what the other tree contains at all.  It discards everything
+the other tree did, declaring 'our' history contains all that happened in it.
+
+theirs;;
+	This is opposite of 'ours'.
+
+subtree[=path];;
+	This option is a more advanced form of 'subtree' strategy, where
+	the strategy makes a guess on how two trees must be shifted to
+	match with each other when merging.  Instead, the specified path
+	is prefixed (or stripped from the beginning) to make the shape of
+	two trees to match.
 
 octopus::
 	This resolves cases with more than two heads, but refuses to do
@@ -33,7 +59,8 @@ ours::
 	merge is always that of the current branch head, effectively
 	ignoring all changes from all other branches.  It is meant to
 	be used to supersede old development history of side
-	branches.
+	branches.  Note that this is different from the -Xours option to
+	the 'recursive' merge strategy.
 
 subtree::
 	This is a modified recursive strategy. When merging trees A and