Adding more godoc for the main beam package.
Adding more godoc for the main beam package.

Change-Id: I9bc6098347e7944cc4822a4db42b4895ee76d9f4
Reviewed-on: https://team-review.git.corp.google.com/143709
Verified-Style: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Verified-BuildAndTest: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Henning Rohde <cc74ff07599c64f633a3f6876da4eb88a647f9b9@google.com>
Reviewed-by: Robert Burke <9780acb0f0129b4ae48b4afb637c166fafc2c02c@google.com>

RyanSkraba/beam,markflyhigh/incubator-beam,charlesccychen/beam,chamikaramj/beam,rangadi/beam,charlesccychen/beam,rangadi/beam,tgroh/beam,charlesccychen/beam,mxm/incubator-beam,lukecwik/incubator-beam,RyanSkraba/beam,apache/beam,markflyhigh/incubator-beam,RyanSkraba/beam,rangadi/incubator-beam,robertwb/incubator-beam,robertwb/incubator-beam,lukecwik/incubator-beam,apache/beam,charlesccychen/incubator-beam,RyanSkraba/beam,robertwb/incubator-beam,chamikaramj/beam,chamikaramj/beam,markflyhigh/incubator-beam,markflyhigh/incubator-beam,mxm/incubator-beam,apache/beam,apache/beam,robertwb/incubator-beam,RyanSkraba/beam,tgroh/incubator-beam,chamikaramj/beam,lukecwik/incubator-beam,robertwb/incubator-beam,lukecwik/incubator-beam,iemejia/incubator-beam,chamikaramj/beam,robertwb/incubator-beam,charlesccychen/beam,markflyhigh/incubator-beam,tgroh/beam,charlesccychen/incubator-beam,tgroh/beam,chamikaramj/beam,chamikaramj/beam,charlesccychen/beam,rangadi/incubator-beam,chamikaramj/beam,markflyhigh/incubator-beam,tgroh/incubator-beam,charlesccychen/incubator-beam,lukecwik/incubator-beam,apache/beam,apache/beam,lukecwik/incubator-beam,rangadi/beam,rangadi/beam,RyanSkraba/beam,robertwb/incubator-beam,apache/beam,rangadi/beam,lukecwik/incubator-beam,apache/beam,chamikaramj/beam,rangadi/incubator-beam,lukecwik/incubator-beam,lukecwik/incubator-beam,robertwb/incubator-beam,robertwb/incubator-beam,apache/beam,RyanSkraba/beam,rangadi/beam,lukecwik/incubator-beam,rangadi/beam,chamikaramj/beam,tgroh/beam,apache/beam,apache/beam,charlesccychen/beam,markflyhigh/incubator-beam,iemejia/incubator-beam,charlesccychen/beam,robertwb/incubator-beam