regexp: document that in *All if n < 0 means return all matches

[odeke-em]

I was just working on a simple login router and while examining regexp.FindAllString, I had to use the common knowledge that usually when you ask for n<0, that means return all the matches. However, this isn't intuitive to everyone. Perhaps let's document it.

Sharing this with my other team members who might not be experienced would definitely lead to confusion as in https://play.golang.org/p/5J4Me7dgNML

[odeke-em]

The package doc for All as recommended in the comments says that if n >=0 it returns at most n matches, but doesn't say anything for if n<0

[gopherbot]

Change https://golang.org/cl/102423 mentions this issue: regexp: document behavior of FindAll* functions when n < 0

[jimmyfrasche]

cc @rsc and @matloob by golang.org/s/owners