Update RenderScript docs: reduction, other API 23/24 changes, cleanup.

Substantive changes:
- Add discussion of reduction kernels.
- Use RS_KERNEL in place of __attribute__((kernel)).
- Discuss "context" special argument.
- Special arguments x, y, z may be of type int.
- Discuss multiple inputs for mapping kernels.
- Mention that script-side global writes are not visible in Java.
- Rewrite part of discussion regarding when to use support library.
- Change minimum device version for support library from Android 2.2 (API level 8)
  to Android 2.3 (API level 9).

Stylistic changes (better conformance with style guide):
- Proper use of <i> (italic) versus <em> (emphasis).
- Proper case for headings.
- Proper pattern for Android version numbers.
- Do not use e.g. or i.e.
- Prefer active rather than passive voice.
- Prefer present rather than future tense.

Miscellaneous minor editorial changes.

Bug: 27299393
Bug: 30000147

Change-Id: Ie0a26152714cf8f619d4914218f7262d675bb25f
(cherry picked from commit 05eef304)