docs: Update LinearLayout JavaDoc comments

Updated top-level description to:

- favor explanations of using XML for setting attributes (instead of using
methods like setGravity()
- add code example
- clarify what "child view" means
- add context around cross-link to layout params
- add context around cross-link Linear Layout guide (this guide focuses on
layout_weight)

Test: docs change only. Tested with doc build
Change-Id: Ifc1fa4cf24e2afa229f1c20b3f725073aff162d7