doc: document experimental features better.

doc: document experimental features better.

* doc/bison.texinfo (Introduction): Say that IELR(1) and canonical
LR(1) are experimental.  Mention Java.  Normally experimental
features probably shouldn't be mentioned in the introduction.
However, if Bison's limitations to LALR(1), C, and C++ are so
important that they should be mentioned here, then it's important
to point out that Bison is beginning to escape those limitations.
Moreover, these particular experimental features have very little
chance of being removed.
* src/getargs.c (usage): Say that IELR(1) and canonical LR(1) are
experimental.