Query size limitation missing in documentation

Hello,

I was looking at https://www.postgresql.org/docs/current/limits.html
(was brought up by a customer) and found we are missing the limitation
of the string of the SQL to be executed (it's 1GB which is the longest
string postgres can hold)

```
/* It's possible we could use a different value for this in frontend code */
#define MaxAllocSize ((Size) 0x3fffffff) /* 1 gigabyte - 1 */
```

Would there be any objections to adding such limitations in the list
from the link above?

Attached in a proposed patch which adds this to the documentation. I
also added a paragraph mentioning why the field and query length are
limited by 1GB.

--
Martín Marqués
It’s not that I have something to hide,
it’s that I have nothing I want you to see