Improve description of loread/lowrite.

Patch by me, reviewed by Tatsuo Ishii.
Fix unescaping of JSON Unicode escapes, especially for non-UTF8.

Per discussion  on -hackers. We treat Unicode escapes when unescaping
them similarly to the way we treat them in PostgreSQL string literals.
Escapes in the ASCII range are always accepted, no matter what the
database encoding. Escapes for higher code points are only processed in
UTF8 databases, and attempts to process them in other databases will
result in an error. \u0000 is never unescaped, since it would result in
an impermissible null byte.
Merge branch 'master' of git://git.postgresql.org/git/postgresql into returning_before_after