bytea functions documentation

Here's my first attempt at a documentation patch :-)

I've created a new section to func.sgml, "Binary String Functions and
Operators", plagiarized shamelessly from the "String Functions and
Operators" section. There were enough differences that it made sense (at
least to me) to give this its own section instead of cramming it in with
normal string functions. This way I could also make the examples
relevant, which is particularly important for bytea.

One thing I think worth mentioning: while documenting the trim()
function I realized that I never implemented the bytea equivalent of
rtrim and ltrim. Therefore, the 'leading' and 'trailing' forms of trim,
available with text, are not available with bytea (I'd be happy to
correct this, but since it would require an initdb, I guess not until
7.3) -- the submitted doc accurately reflects this.

I will look for other areas of the docs that need mention of bytea, but
any guidance would be much appreciated.

regards,

-- Joe