Patch to document base64 encoding

Hi,

Doc patch, against master. Documents encode() and decode() base64
format.

Builds for me.

Attached: doc_base64_v1.patch

References RFC2045 section 6.8 to define base64.

Because encode() and decode() show up in both the string
functions section and the binary string functions section
I documented in only the string functions section and hyperlinked
"base64" in both sections to the new text.

Note that XML output can also generate base64 data. I suspect
this is done via the (different, src/common/base64.c)
pg_b64_encode() function which does not limit line length.
In any case this patch does not touch the XML documentation.

Regards,

Karl <kop(at)meme(dot)com>
Free Software: "You don't pay back, you pay forward."
-- Robert A. Heinlein