update fig-vars doc

Signed-off-by: harupy <17039389+harupy@users.noreply.github.com>

Author: harupy

docs/source/examples/fig-vars.rst

@@ -4,6 +4,8 @@ fig-vars
 Single
 ~~~~~~
 
+With `fig-vars` option, you can render a plotly figure assigned in a variable.
+
 .. plotly::
    :fig-vars: fig1
 
@@ -15,6 +17,8 @@ Single
 Multiple
 ~~~~~~~~
 
+You can specify multiple variables.
+
 .. plotly::
    :fig-vars: fig1, fig2