flowchart updated

I have seriously updated the backend flowchart.  In fact, the name is
gone, and it is now called "How PostgreSQL Processes a Query".

I have added several paragraphs describing the flow of data from one
module to another, and have added a section describing 'exactly' what is
stored in shared memory, and how it is used.

If you have current source tree, point your browser at:

    file:/usr/local/pgsql/src/tools/backend

or the proper directory for your system.  You can also view it on our
www site from the docs page.

I am interested in comments, suggestions, and any possible errors.

This will certainly help people get started coding, because at least
they will understand how all the parts fit together.

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