Deprecate references to extdata

Since we're moving toward using Hiera, I've updated the docs. Clever, eh?
There might also be the odd grammatical fix in this commit,too.
Sneak in an extra grammar fix or two.

You know, while @ajlanghorn's in there already.
Correct smushed-together lines.

@ajlanghorn says it was all Vim's fault.
Sneak in another grammar fix.

You know, while @fatbusinessman's in there already.
Merge pull request #39 from alphagov/deprecate-extdata-references

Deprecate references to extdata