internal/frontend: drop docs after all/all
internal/frontend: drop docs after all/all

If there is more than one Documentation in a unit but the first is
all/all, ignore the others.

Also, consolidate doc mods into one function.

Change-Id: I9ec7551d39a4e68e8a4a1483f928d288d77e1e55
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/305989
Trust: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Run-TryBot: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite