Update documentation for genrule and sdk_library

1. Documentation for genrule and sdk_library was missing a period, which
has been added now. For
genrule, Before:
https://android-build.googleplex.com/builds/submitted/7594870/linux/latest/view/genrule.html,
After:
https://x20web.corp.google.com/users/sp/spandandas/soong_docs/genrule.html

2. Documentation for genrule was missing definition of $(locations),
which has been added now

Bug: 194231221
Test: m soong_docs
Change-Id: Ia367f13055c110d92a491a9c1b1af4113f66b296