blame-options.txt: document --first-parent option

blame/annotate have supported --first-parent since commit 95a4fb0eac
("blame: handle --first-parent"). This adds a blurb on that option to
the documentation.

Signed-off-by: Raymond E. Pasco <ray@ameretat.dev>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/blame-options.txt

@@ -36,6 +36,12 @@ include::line-range-format.txt[]
 	START.  `git blame --reverse START` is taken as `git blame
 	--reverse START..HEAD` for convenience.
 
+--first-parent::
+	Follow only the first parent commit upon seeing a merge
+	commit. This option can be used to determine when a line
+	was introduced to a particular integration branch, rather
+	than when it was introduced to the history overall.
+
 -p::
 --porcelain::
 	Show in a format designed for machine consumption.