[site_nav] Document 1000-entry limit for friends pages

[pne]

FAQ #148 mentions that LASTN only goes back 75 entries and that FRIENDS only goes back two weeks.

However, apparently there is an additional limit to the FRIENDS view: it'll only show 1000 entries, even if that doesn't go back two whole weeks. (This was discussed most recently in a thread in helpscreening.)

I suggest that this limit be added to FAQ 148.