git-blame(1): mention options in the synopsis and advertise pickaxe

Inspired by the cvs annotate documentation improve and expand the man page
to also mention the limitations of file annotations. Since people coming
from the SVN/CVS world might first look here, also briefly advertise how
the pickaxe interface makes it easy to go beyond these limitation.
Signed-off-by: NJonas Fonseca <fonseca@diku.dk>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Documentation/git-blame.txt

@@ -3,21 +3,38 @@ git-blame(1)
 
 NAME
 ----
-git-blame - Blame file lines on commits
+git-blame - Show what revision and author last modified each line of a file
 
 SYNOPSIS
 --------
-git-blame file [options] file [revision]
+'git-blame' [-c] [-l] [-t] [-S <revs-file>] [--] <file> [<rev>]
 
 DESCRIPTION
 -----------
-Annotates each line in the given file with information from the commit
-which introduced the line. Start annotation from the given revision.
+
+Annotates each line in the given file with information from the revision which
+last modified the line. Optionally, start annotating from the given revision.
+
+This report doesn't tell you anything about lines which have been deleted or
+replaced; you need to use a tool such as gitlink:git-diff[1] or the "pickaxe"
+interface briefly mentioned in the following paragraph.
+
+Apart from supporting file annotation, git also supports searching the
+development history for when a code snippet occured in a change. This makes it
+possible to track when a code snippet was added to a file, moved or copied
+between files, and eventually deleted or replaced. It works by searching for
+a text string in the diff. A small example:
+
+-----------------------------------------------------------------------------
+$ git log --pretty=oneline -S'blame_usage'
+5040f17eba15504bad66b14a645bddd9b015ebb7 blame -S <ancestry-file>
+ea4c7f9bf69e781dd0cd88d2bccb2bf5cc15c9a7 git-blame: Make the output
+-----------------------------------------------------------------------------
 
 OPTIONS
 -------
 -c, --compatibility::
-	Use the same output mode as git-annotate (Default: off).
+	Use the same output mode as gitlink:git-annotate[1] (Default: off).
 
 -l, --long::
 	Show long rev (Default: off).
@@ -26,7 +43,7 @@ OPTIONS
 	Show raw timestamp (Default: off).
 
 -S, --rev-file <revs-file>::
-	Use revs from revs-file instead of calling git-rev-list.
+	Use revs from revs-file instead of calling gitlink:git-rev-list[1].
 
 -h, --help::
 	Show help message.