rev-list documentation: clarify the two parts of history simplification

One set of options and parameters determine what commits are involved in
the simplification process, and another set of options determine how the
simplification is done.  Clarify their distinction at the beginning.

Signed-off-by: Santi Béjar <santi@agolina.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/rev-list-options.txt

@@ -285,8 +285,52 @@ See also linkgit:git-reflog[1].
 History Simplification
 ~~~~~~~~~~~~~~~~~~~~~~
 
-When optional paths are given, 'git-rev-list' simplifies commits with
-various strategies, according to the options you have selected.
+Sometimes you are only interested in parts of the history, for example the
+commits modifying a particular <path>. But there are two parts of
+'History Simplification', one part is selecting the commits and the other
+is how to do it, as there are various strategies to simplify the history.
+
+The following options select the commits to be shown:
+
+<paths>::
+
+	Commits modifying the given <paths> are selected.
+
+--simplify-by-decoration::
+
+	Commits that are referred by some branch or tag are selected.
+
+Note that extra commits can be shown to give a meaningful history.
+
+The following options affect the way the simplification is performed:
+
+Default mode::
+
+	Simplifies the history to the simplest history explaining the
+	final state of the tree. Simplest because it prunes some side
+	branches if the end result is the same (i.e. merging branches
+	with the same content)
+
+--full-history::
+
+	As the default mode but does not prune some history.
+
+--dense::
+
+	Only the selected commits are shown, plus some to have a
+	meaningful history.
+
+--sparse::
+
+	All commits in the simplified history are shown.
+
+--simplify-merges::
+
+	Additional option to '--full-history' to remove some needless
+	merges from the resulting history, as there are no selected
+	commits contributing to this merge.
+
+A more detailed explanation follows.
 
 Suppose you specified `foo` as the <paths>.  We shall call commits
 that modify `foo` !TREESAME, and the rest TREESAME.  (In a diff