Better documentation in pins_arduino.h [imported]

This is [Issue 1055](http://code.google.com/p/arduino/issues/detail?id=1055) moved from a Google Code project.
Added by 2012-09-30T08:09:30.000Z by [wes...@gmail.com](http://code.google.com/u/103342639718054305536/).
Please review that bug for more context and additional comments, but update this bug.

Original labels: Type-Enhancement, Priority-Medium
### Original description

**What steps will reproduce the problem?**
1. There have been requests for a tutorial on modifying pins_arduino.h to meet the needs of custom-designed boards.
2. pins_arduino.h is not well documented internally.

The attached diffs add a bunch of comments (only! No actual code changes!) to variants/standard/pins_arduino.h explaining what it does and approximately how it works. This should make it a bit less confusing for a moderately skilled programmer to make their own variant.