Update examples for string-related functions.

In the example for decode(), show the bytea result in hex format,
since that's now the default.  Use an E'' string in the example for
quote_literal(), so that it works regardless of the
standard_conforming_strings setting.  On the functions-for-binary-strings
page, leave the examples as-is for readability, but add a note pointing out
that they are shown in escape format.  Per comments from Thom Brown.

Also, improve the description for encode() and decode() a tad.

Backpatch to 9.0, where bytea_output was introduced.