Document filter parameter

[MisterDA]

Fix #340.

[craigfe]

One unrelated remaining failure due to lwt.5.5.0:

#=== ERROR while compiling lwt.5.5.0 ==========================================#
# context     2.0.10 | win32/x86_64 | ocaml-variants.4.13.1+mingw64c | git+https://github.com/fdopen/opam-repository-mingw.git#opam2
# path        D:/a/alcotest/alcotest/_opam/.opam-switch/build/lwt.5.5.0
# command     D:\a\alcotest\alcotest\_opam\bin\dune.exe build -p lwt -j 2
# exit-code   1
# env-file    D:/.opam/log/lwt-1664-b53f9e.env
# output-file D:/.opam/log/lwt-1664-b53f9e.out
### output ###
# File "_build/.dune/default/src/core/dune", line 8, characters 18-24:
# 8 |  (libraries bytes result seq)
#                       ^^^^^^
# Error: Library "result" not found.
# -> required by library "lwt" in _build/default/src/core
# -> required by _build/default/META.lwt
# -> required by _build/install/default/lib/lwt/META
# -> required by _build/default/lwt.install
# -> required by alias install

I'm not sure why the result dependency from lwt.5.5.0 is not included on win32, given that the dune files seem to require it. Looks like this will no longer be an issue once lwt.5.6.0 is out, since it's dropping the dependency on result completely (ocsigen/lwt#925). CC @raphael-proust.