Fix detailed guide translation keys

These translation do not belong under `document.type.detailed_guidance`,
 that is reserved for the plural-form translations for the term
 "Detailed Guidance" only.
Remove unsed translation key

There will be others, but the presence of this one makes it harder to
clean up the other "see_all" translation keys
Correctly nest "see all" translations

Pluralised translation terms such as those under "document.type" should
only ever contain pluralisation keys, otherwise the import/export code
does not recognise them as pluralised and cannot correctly generate the
correct plural forms when regenerating the locale files.
Regenerate translation files to add missing keys

Now that the pluralised translation terms have only pluralisation keys,
we can safely regenerate the files to correct any anomalies.
Add note to docs

These docs are not widely read, but it's better to mention it here than
not at all.
Update README to reflect pluralisation fixes
Merge pull request #1733 from alphagov/locale-fixes

Repair locale files by moving non-plural-form keys out of pluralised translations