Fix documentation of --first-parent in git-log and copy it to git-rev-list

Credit goes to Avi Kivity for noticing the lack of description in
rev-list manual page.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-log.txt

@@ -43,8 +43,12 @@ include::diff-options.txt[]
 
 --first-parent::
 	Follow only the first parent commit upon seeing a merge
-	commit.  This  option gives a better overview of the
-	evolution of a particular branch.
+	commit.  This option can give a better overview when
+	viewing the evolution of a particular topic branch,
+	because merges into a topic branch tend to be only about
+	adjusting to updated upstream from time to time, and
+	this option allows you to ignore the individual commits
+	brought in to your history by such a merge.
 
 -g, \--walk-reflogs::
 	Show commits as they were recorded in the reflog. The log contains

Documentation/git-rev-list.txt

@@ -15,6 +15,7 @@ SYNOPSIS
 	     [ \--min-age=timestamp ]
 	     [ \--sparse ]
 	     [ \--no-merges ]
+	     [ \--first-parent ]
 	     [ \--remove-empty ]
 	     [ \--full-history ]
 	     [ \--not ]
@@ -256,6 +257,15 @@ limiting may be applied.
 
 	Do not print commits with more than one parent.
 
+--first-parent::
+	Follow only the first parent commit upon seeing a merge
+	commit.  This option can give a better overview when
+	viewing the evolution of a particular topic branch,
+	because merges into a topic branch tend to be only about
+	adjusting to updated upstream from time to time, and
+	this option allows you to ignore the individual commits
+	brought in to your history by such a merge.
+
 --not::
 
 	Reverses the meaning of the '{caret}' prefix (or lack thereof)