line-range-format.txt: document -L/RE/ relative search

Option -L/RE/ of blame/log now searches relative to the previous -L
range, if any. Document this.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/line-range-format.txt

@@ -9,7 +9,9 @@ absolute line number (lines count from 1).
 - /regex/
 +
 This form will use the first line matching the given
-POSIX regex.  If <end> is a regex, it will search
+POSIX regex. If <start> is a regex, it will search from the end of
+the previous `-L` range, if any, otherwise from the start of file.
+If <end> is a regex, it will search
 starting at the line given by <start>.
 +