[#969] Add instructions for package patch upgrades
[#969] Edit the upgrading to a patch release docs

- Group the *Upgrading to a patch version* and *Upgrading a 1.X Package
  Install to CKAN 2.0* section headings under a top-level *Upgrading a
  package install* heading (this is just so that only one *Upgrading a
  package install* heading appears in the table of contents)

- Edit the first couple of paragraphs of *Upgrading to a patch version*
  a little

- Say CKAN config file not ini file

- Use |apache| and |nginx| to get correct capitalisation of those names
Merge branch '969-upgrade-patch-instructions'