doc: catch up with the current display of cex

Unfortunately I found no way to use the ↳ glyph in Texinfo, so I used
@arrow{} instead, which has a different width, so we have to have all
the examples doubled, once for TeX, another for the rest of the world.

* doc/bison.texi: Use the current display in the examples.
* doc/calc.y, doc/ids.y, doc/if-then-else.y, doc/sequence.y: New.

doc/sequence.y

@@ -0,0 +1,10 @@
+%%
+sequence:
+  %empty
+| maybeword
+| sequence "word"
+;
+maybeword:
+  %empty
+| "word"
+;