Rename the docs .txt files to .rst.

This renames all our docs to .rst, which will give us
syntax-highlighting during code review, and allow us to remove the vim
syntax markers.
Strip out the vim formatting markers in the docs.

Now that we have .rst files, we no longer need the vim markers that tell
vim what format the files are.

This is being done separately from the rename change so that git doesn't
get confused and lose the rename information.