docs: Update EditText JavaDoc comments

rework the edit text introduction and overview.

- adds an example of a basic configuration
- discuss the inputType attribute
- cross link to relevant classes & methods
- Rework the edit text introduction and overview
- wrap guide link in a p to emphasize
- emphasize specifying the inputType is required

Test: docs only change. Verified with doc test build
Change-Id: I78d8c861c51117bfd61e8c37ad83854a2aeec967