Fix inappropriate uses of PG_GETARG_UINT32()

I noticed that the chr() function uses PG_GETARG_UINT32() to get its
argument, even though the argument is a (signed) int. So you get some
slightly silly behavior like this:

=> select chr(-333);
ERROR: 54000: requested character too large for encoding: -333

The attached patch fixes this by accepting the argument using
PG_GETARG_INT32(), doing some checks, and then casting it to unsigned
for the rest of the code.

The patch also fixes another inappropriate use in an example in the
documentation. These two were the only inappropriate uses I found,
after we had fixed a few recently.