Documentation: clarify effects of -- <path> arguments

'git log -- <path>' does not "show commits that affect the specified
paths" in a literal sense unless --full-history is given (for example,
a file that only existed on a side branch will turn up no commits at
all!).

Reword it to specify the actual intent of the filtering, and point to
the "History Simplification" section.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-log.txt

@@ -69,10 +69,13 @@ produced by --stat etc.
 	its size is not included.
 
 [\--] <path>...::
-	Show only commits that affect any of the specified paths. To
+	Show only commits that are enough to explain how the files
-	prevent confusion with options and branch names, paths may need
+	that match the specified paths came to be.  See "History
-	to be prefixed with "\-- " to separate them from options or
+	Simplification" below for details and other simplification
-	refnames.
+	modes.
++
+To prevent confusion with options and branch names, paths may need to
+be prefixed with "\-- " to separate them from options or refnames.
 
 include::rev-list-options.txt[]