Allow walking multiple root directories (#4109)

Co-authored-by: Martin Sabathier <martin.sabathier.ext@corys.fr>
Co-authored-by: Junegunn Choi <junegunn.c@gmail.com>

man/man1/fzf.1

@@ -1005,8 +1005,8 @@ Determines the behavior of the built-in directory walker that is used when
 .br
 
 .TP
-.B "\-\-walker\-root=DIR"
-The root directory from which to start the built-in directory walker.
+.B "\-\-walker\-root=DIR [...]"
+List of directory names to start the built-in directory walker.
 The default value is the current working directory.
 
 .TP