Fix and clarify pin descriptions

[MHassanNadeem]

1. Fixed typos. Changed A0-A1 to A0-A5 and D0-D1 to D0-D7.
2. Moved descriptions of alternate pin functions to appropriate locations.

[monkbroc]

Thanks. I'll include your suggestions shortly.

[technobly]

Thanks for the contribution @Hassan-n

@monkbroc please hold off on merging this.

All changes to the datasheets should be staged here: #19

And they should be reviewed and approved by @mohitbhoite and/or me. Thanks!

[technobly]

A bit of confusion here... essentially the changes did make it into master, but through a small re-write I did in #463 Thanks @MHassanNadeem !