Change error to warning and increase thresholds of tsearch

Hi,

I was looking at Todo item:/Consider changing error to warning for strings larger than one megabyte/
and after going through existing mails and suggestions. I would like to propose a patch for tsearch to change error into warning for string larger than one mb and also increase word and position limits.

I've checked operations select/insertion/index, which worked fine without any error (except for the warning as intended).

Thoughts: I am not really sure why was it proposed in the mail to decrease len/MAXSTRLEN.
> You could decrease len in WordEntry to 9 (512 characters) and increase
> pos to 22 (4 Mb). Don't forget to update MAXSTRLEN and MAXSTRPOS
> accordingly.

I'm attaching a patch herewith. I will be glad to get some feedback on this.

Thanks,
Ankit