This patch adds a table summarizing the history or MI versions:

- The version number
- Which GDB version introduced it
- Breaking changes compared to the previous version

The goal of the table is to help writers of front ends know which
version of MI they can use with a given GDB version.  It will also help
them update their code to work against a newer MI version.

Right now, we just have 1 and 2, but we expect to add an entry for 3
soon.  I did a bit of archelogy and reverse engineering of the code to
come up with the breaking changes for MI 2.

I did some changes to the text around it, some things that I thought
needed to be clarified, seemed a bit dated or seemed just wrong
(especially "Apart from mi0, new versions of @value{GDBN} will not
support old versions of MI").

gdb/doc/ChangeLog:

	* gdb.texinfo (GDB/MI Development and Front Ends): Add table of
	MI versions.  Update text around it.