Reattach filter's docstring

[Sacha0]

#18839 moved the docstring-bearing filter method into module Iterators. Module Iterators does not import filter from Base, so filter's docstring no longer attaches to Base.filter:

julia> VERSION
v"0.6.0-dev.1054"

help?> filter
search: filter filter! PollingFileWatcher fieldtype

  No documentation found.

  Base.filter is a Function.

  #7 methods for generic function "filter":
  filter(f, a::Array{T<:Any,1}) at array.jl:1656
  filter(f, Bs::BitArray) at bitarray.jl:2014
  filter(f, As::AbstractArray) at array.jl:1641
  filter(f, d::Associative) at associative.jl:211
  filter(f, s::Set) at set.jl:171
  filter(f, s::AbstractString) at strings/basic.jl:449
  filter(flt, itr) at deprecated.jl:49

This pull request attaches filter's docstring to filter's AbstractArray fallback in base/array.jl. Best!