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